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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.