Automatic merge from submit-queue (batch tested with PRs 50254, 50174, 50179)

kubeadm: Add back labels for the Static Pod control plane

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

This Labels section has been removed now for a short period during the v1.8 dev cycle, but I found a good use-case for it; namely filtering Mirror Pods by the `component=kube-*` label when waiting for the self-hosted control plane to come up after an upgrade. It's not _really_ neccessary, but nice to have.

Also noticed the lack of coverage for this func, so added a small unit test.

**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #

Dependency for: https://github.com/kubernetes/kubernetes/pull/48899

**Special notes for your reviewer**:

**Release note**:

```release-note
NONE
```
@kubernetes/sig-cluster-lifecycle-pr-reviews @dmmcquay @timothysc @mattmoyer