Automatic merge from submit-queue (batch tested with PRs 63314, 63884, 63799, 63521, 62242). 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: Restructure internal config usage and fix bugs
**What this PR does / why we need it**:
- Moves the generic LoadYAML function from the versioned, external API package to a helper library so it can be consumed more easily
- Makes the upgrading code use the internal version of the API (which always should be used anyway)
- Moves all config-loading code to `configutil`, together with the migration code needed. This way we have everything in one centralized place, instead of duplicating that logic N times.
- Makes `kubeadm init` use `configutil` for the reasons mentioned above.
This PR is needed in order to support multiple external API groups (like v1alpha2)
**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Part of https://github.com/kubernetes/community/pull/2131
**Special notes for your reviewer**:
This PR depends on:
- https://github.com/kubernetes/kubernetes/pull/63782
- https://github.com/kubernetes/kubernetes/pull/63783
**Please review only the last (third) commit**
**Release note**:
```release-note
NONE
```
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