Automatic merge from submit-queue (batch tested with PRs 63865, 57849, 63932, 63930, 63936). 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>.

Implement `kubeadm init diff`

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

Some users want to see the changes `kubeadm` woulda apply before actually running `kubeadm upgrade apply`. This shows the changes that will be made to the static pod manifests before applying them. This is a narrower case than `kubeadm upgrade apply --dry-run`, which specifically focuses on the static pod manifests.

**Which issue(s) this PR fixes**:
Part of [kubeadm/489](https://github.com/kubernetes/kubeadm/issues/489#issuecomment-388974795)

**Special notes for your reviewer**:

**Release note**:

```release-note
adds the `kubeadm upgrade diff` command to show how static pod manifests will be changed by an upgrade.
```