Automatic merge from submit-queue (batch tested with PRs 62726, 60085, 62583). 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>.
Refactor kubeadm api validation.
**What this PR does / why we need it**:
This PR refactor kubeadm api validation to use field json name. When users get a validation error, they can easily know which field is invalid in their config files.
**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Fixes #
**Special notes for your reviewer**:
@fabriziopandini I remember we mentioned this refactoring in some comment. I didn't see this change was done, so I send this PR to address this.
**Release note**:
```release-note
NONE
```
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