Automatic merge from submit-queue (batch tested with PRs 52520, 52033, 53626, 50478). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Add 'ETCD_DOCKER_REPOSITORY' env var override to 'cluster/' scripts.

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

This allows the etcd docker registry that is currently hard coded to `gcr.io/google_containers/etcd` in the `etcd.manifest` template to be overridden.  This can be used to test new versions of etcd with kubernetes that have not yet been published to `gcr.io/google_containers/etcd` and also enables cluster operators that use the 'cluster/' scripts to manage the etcd images used by their cluster in an internal repository.

Example usage:

```
$ export ETCD_DOCKER_REPOSITORY=gcr.io/jpbetz-public-images/etcd 
$ export ETCD_IMAGE=3.0.17 
$ cluster/kube-up.sh
...
$ gcloud compute ssh kubernetes-master
~ $ sudo cat /etc/kubernetes/manifests/etcd.manifest
{
"apiVersion": "v1",
"kind": "Pod",
"metadata": {
  "name":"etcd-server",
  ...
},
"spec":{
"hostNetwork": true,
"containers":[
    {
    "name": "etcd-container",
    "image": "gcr.io/jpbetz-public-images/etcd:3.0.17",
...
```
**Release note**:
```release-note
```