Automatic merge from submit-queue. 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>.
removes k8s.io/kubernetes/pkg/api dependency from the webhook plugin.
**What this PR does / why we need it**: removes `k8s.io/kubernetes/pkg/api` dependency from `webhook` plugin. The runtime.scheme can be injected to the webhook from the plugin initializer.
**Release note**:
```
NONE
```
Automatic merge from submit-queue. 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>.
Move initializer plugin to the generic apiserver
* Moves `k8s.io/kuberentes/plugin/pkg/admission/initialization` to `k8s.io/apiserver/pkg/admission/plugin/initialization`
* Moves `k8s.io/kubernetes/pkg/kubeapiserver/admission/configuration` to `k8s.io/apiserver/pkg/admission/configuration`
* The initializer plugin used to depend on `k8s.io/kubernetes` because it does a type assertion of `api.Pod`. It tries to skip mirror pod. I converted that code to use the generic accessor pattern.
Automatic merge from submit-queue. 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>.
moves admission.v1alpha1.NewAdmissionReview method to webhook plugin
**What this PR does / why we need it**:
this is necessary, because the webhook plugin will be moved down to apiserver.
**Release note**:
```
NONE
```
