Automatic merge from submit-queue (batch tested with PRs 40297, 41285, 41211, 41243, 39735)

cluster/gce: Add env var to enable apiserver basic audit log.

For now, this is focused on a fixed set of flags that makes the audit
log show up under /var/log/kube-apiserver-audit.log and behave similarly
to /var/log/kube-apiserver.log. Allowing other customization would
require significantly more complex changes.

Audit log rotation is handled the same as for `kube-apiserver.log`.

**What this PR does / why we need it**:

Add a knob to enable [basic audit logging](https://kubernetes.io/docs/admin/audit/) in GCE.

**Which issue this PR fixes**:

**Special notes for your reviewer**:

We would like to cherrypick/port this to release-1.5 also.

**Release note**:
```release-note
The kube-apiserver [basic audit log](https://kubernetes.io/docs/admin/audit/) can be enabled in GCE by exporting the environment variable `ENABLE_APISERVER_BASIC_AUDIT=true` before running `cluster/kube-up.sh`. This will log to `/var/log/kube-apiserver-audit.log` and use the same `logrotate` settings as `/var/log/kube-apiserver.log`.
```