Automatic merge from submit-queue

Remove Initializers from admission-control in kubernetes-master charm for pre-1.7

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

This fixes a problem with the kubernetes-master charm where kube-apiserver never comes up:

```
failed to initialize admission: Unknown admission plugin: Initializers
```

The Initializers plugin does not exist before Kubernetes 1.7. The charm needs to support 1.6 as well.

**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #47062

**Special notes for your reviewer**:

This fixes a problem introduced by https://github.com/kubernetes/kubernetes/pull/36721

**Release note**:

```release-note
Remove Initializers from admission-control in kubernetes-master charm for pre-1.7
```