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.
6154a9e16c