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>.
Add roundtrip, defaulting, upgrading and validation unit tests for the kubeadm API types
**What this PR does / why we need it**:
Follows up from https://github.com/kubernetes/kubernetes/pull/63799, as well as net-new unit testing for our serialization/deserialization package. This tests our API machinery pretty much end to end.
This is more important now given we now support two external types: https://github.com/kubernetes/kubernetes/pull/63788
**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 kubernetes/community#2131
**Special notes for your reviewer**:
**Release note**:
```release-note
NONE
```
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