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>.

kubeadm: imagePullPolicy option in init config

**What this PR does / why we need it**:
This PR adds `imagePullPolicy` option to the `kubeadm init` configuration file.

The new `imagePullPolicy` option is forwarded to the generated kubelet static pods for etcd, kube-apiserver, kube-controller-manager and kube-scheduler. This option allows for precise image pull policy specification for master nodes and thus for more tight control over images. It is useful in CI environments and in environments, where the user has total control over master VM templates (thus, the master VM templates can be preloaded with the required Docker images for the control plane services).

**Special notes for your reviewer**:

/cc @kubernetes/sig-cluster-lifecycle-pr-reviews
/area kubeadm
/assign @luxas

**Release note**:

```release-note
kubeadm: New "imagePullPolicy" option in the init configuration file, that gets forwarded to kubelet static pods to control pull policy for etcd and control plane images.
```