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>.
Make healthchecks skippable, and check masters only
**What this PR does / why we need it**:
Previously kubeadm would abort if _any_ node was not Ready. This is obviously infeasible in a non-trivial (esp. baremetal) cluster.
This PR makes two changes:
- Allows kubeadm healthchecks to be selectively skipped (made non-fatal) with --ignore-checks-errors.
- Check only that the *master* nodes are Ready.
**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 kubernetes/kubeadm#539
**Special notes for your reviewer**:
Builds on #56072
**Release note**:
```release-note
kubeadm health checks can also be skipped with `--ignore-checks-errors`
```
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