Fix three issues with the fluentd-gcp liveness probe:

h1. STUCK_THRESHOLD_SECONDS was overridden by LIVENESS_THRESHOLD_SECONDS
if defined

Probably a copy/paste issue introduced in edf1ffc0

h1. `[[` is [a bashism](https://stackoverflow.com/a/47576482), and will always failed when called with `/bin/sh`

Introduced by a844523c

Given that we call the liveness probe with `/bin/sh`, we cannot use the
double-bracketed `[[` syntax for test, as it is not POSIX-compliant and
will throw an error.

Annoyingly, even through it prints an error, `sh` returns with exit code 0
in this case:

```bash
root@fluentd-7mprs:/# sh liveness.sh
liveness.sh: 8: liveness.sh: [[: not found
liveness.sh: 15: liveness.sh: [[: not found
root@fluentd-7mprs:/# echo $?
0
```

Which means the liveness probe is considered successful by Kubernetes,
despite failing to test things as it was intended. This is also
probably the reason why this bug wasn't reported sooner :)

Thankfully, the test in this case can just as easily be written as
POSIX-compliant as it doesn't use any bash-specific features within the
`[[` block.

h1. Buffers are transient and cannot be relied upon for monitoring

Finally, after fixing the above issue, we started seeing the fluentd
containers being restarted very often, and found an issue with the
underlying logic of the liveness probe.

The probe checks that the pod is still alive by running the following
command:

`find /var/log/fluentd-buffers -type f -newer /tmp/marker-stuck -print -quit`

This checks if any _regular_ file exists under `/var/log/fluentd-buffers`
that is more recent than a predetermined time, and will return an empty
string otherwise.

The issue is that these buffers are temporary and volatile, they get created and
deleted constantly. Here is an example of running that check every second on a
running fluentd:

```
root@fluentd-eks-playground-jdc8m:/# LIVENESS_THRESHOLD_SECONDS=${LIVENESS_THRESHOLD_SECONDS:-300};
root@fluentd-eks-playground-jdc8m:/# STUCK_THRESHOLD_SECONDS=${LIVENESS_THRESHOLD_SECONDS:-900};
root@fluentd-eks-playground-jdc8m:/# touch -d "${STUCK_THRESHOLD_SECONDS} seconds ago" /tmp/marker-stuck;
root@fluentd-eks-playground-jdc8m:/# touch -d "${LIVENESS_THRESHOLD_SECONDS} seconds ago" /tmp/marker-liveness;
root@fluentd-eks-playground-jdc8m:/# while true; do date ; find /var/log/fluentd-buffers -type f -newer /tmp/marker-stuck -print -quit ; sleep 1 ; done
Fri Feb 22 10:52:57 UTC 2019
Fri Feb 22 10:52:58 UTC 2019
/var/log/fluentd-buffers/kubernetes.system.buffer/buffer.b5827964ccf4c7004103c3fa7c8533f85.log
Fri Feb 22 10:52:59 UTC 2019
/var/log/fluentd-buffers/kubernetes.system.buffer/buffer.b5827964ccf4c7004103c3fa7c8533f85.log
Fri Feb 22 10:53:00 UTC 2019
Fri Feb 22 10:53:01 UTC 2019
/var/log/fluentd-buffers/kubernetes.system.buffer/buffer.b5827964fb8b2eedcccd2763ea7775cc2.log
Fri Feb 22 10:53:02 UTC 2019
/var/log/fluentd-buffers/kubernetes.system.buffer/buffer.b5827964fb8b2eedcccd2763ea7775cc2.log
Fri Feb 22 10:53:03 UTC 2019
Fri Feb 22 10:53:04 UTC 2019
Fri Feb 22 10:53:05 UTC 2019
Fri Feb 22 10:53:06 UTC 2019
/var/log/fluentd-buffers/kubernetes.system.buffer/buffer.b5827965564883997b673d703af54848b.log
Fri Feb 22 10:53:07 UTC 2019
/var/log/fluentd-buffers/kubernetes.system.buffer/buffer.b5827965564883997b673d703af54848b.log
Fri Feb 22 10:53:08 UTC 2019
/var/log/fluentd-buffers/kubernetes.system.buffer/buffer.b5827965564883997b673d703af54848b.log
Fri Feb 22 10:53:09 UTC 2019
Fri Feb 22 10:53:10 UTC 2019
Fri Feb 22 10:53:11 UTC 2019
Fri Feb 22 10:53:12 UTC 2019
Fri Feb 22 10:53:13 UTC 2019
Fri Feb 22 10:53:14 UTC 2019
Fri Feb 22 10:53:15 UTC 2019
Fri Feb 22 10:53:16 UTC 2019
```

We can see buffers being created, then disappearing. The LivenessProbe running
under these conditions has a ~50% chance of failing, despite fluentd being
perfectly happy.

I believe that check is probably ok for fluentd installs using large
amounts of buffers, in which case the liveness probe will be correct more
often than not, but fluentd installs that use buffering less intensively
will be negatively impacted by this.

My solution to fix this is to check the last updated time of buffering
_folders_ within `/var/log/fluentd_buffers`. These _do_ get updated when
buffers are created, and do not get deleted as buffers are emptied,
making them the perfect candidate for our use.

Here's an example with the `-d` flag for directories:
```
root@fluentd-eks-playground-jdc8m:/# while true; do date ; find /var/log/fluentd-buffers -type d -newer /tmp/marker-stuck -print -quit ; sleep 1 ; done
Fri Feb 22 10:57:51 UTC 2019
/var/log/fluentd-buffers/kubernetes.system.buffer
Fri Feb 22 10:57:52 UTC 2019
/var/log/fluentd-buffers/kubernetes.system.buffer
Fri Feb 22 10:57:53 UTC 2019
/var/log/fluentd-buffers/kubernetes.system.buffer
Fri Feb 22 10:57:54 UTC 2019
/var/log/fluentd-buffers/kubernetes.system.buffer
Fri Feb 22 10:57:55 UTC 2019
/var/log/fluentd-buffers/kubernetes.system.buffer
Fri Feb 22 10:57:56 UTC 2019
/var/log/fluentd-buffers/kubernetes.system.buffer
Fri Feb 22 10:57:57 UTC 2019
/var/log/fluentd-buffers/kubernetes.system.buffer
Fri Feb 22 10:57:58 UTC 2019
/var/log/fluentd-buffers/kubernetes.system.buffer
Fri Feb 22 10:57:59 UTC 2019
/var/log/fluentd-buffers/kubernetes.system.buffer
Fri Feb 22 10:58:00 UTC 2019
/var/log/fluentd-buffers/kubernetes.system.buffer
Fri Feb 22 10:58:01 UTC 2019
/var/log/fluentd-buffers/kubernetes.system.buffer
Fri Feb 22 10:58:02 UTC 2019
/var/log/fluentd-buffers/kubernetes.system.buffer
Fri Feb 22 10:58:03 UTC 2019
/var/log/fluentd-buffers/kubernetes.system.buffer
```

And example of the directory being updated as new buffers come in:
```
root@fluentd-eks-playground-jdc8m:/# ls -lah /var/log/fluentd-buffers/kubernetes.system.buffer
total 0
drwxr-xr-x 2 root root  6 Feb 22 11:17 .
drwxr-xr-x 3 root root 38 Feb 22 11:14 ..
root@fluentd-eks-playground-jdc8m:/# ls -lah /var/log/fluentd-buffers/kubernetes.system.buffer
total 16K
drwxr-xr-x 2 root root  224 Feb 22 11:18 .
drwxr-xr-x 3 root root   38 Feb 22 11:14 ..
-rw-r--r-- 1 root root 1.8K Feb 22 11:18 buffer.b58279be6e21e8b29fc333a7d50096ed0.log
-rw-r--r-- 1 root root  215 Feb 22 11:18 buffer.b58279be6e21e8b29fc333a7d50096ed0.log.meta
-rw-r--r-- 1 root root  429 Feb 22 11:18 buffer.b58279be6f09bdfe047a96486a525ece2.log
-rw-r--r-- 1 root root  195 Feb 22 11:18 buffer.b58279be6f09bdfe047a96486a525ece2.log.meta
root@fluentd-eks-playground-jdc8m:/# ls -lah /var/log/fluentd-buffers/kubernetes.system.buffer
total 0
drwxr-xr-x 2 root root  6 Feb 22 11:18 .
drwxr-xr-x 3 root root 38 Feb 22 11:14 ..
```