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
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