We allow a kubeadm user to use an external CA by checking to see if ca.key is missing and skipping cert checks and kubeconfig generation if ca.key is missing.
Automatic merge from submit-queue
kubeadm: Remove some old comments
**What this PR does / why we need it**:
Removes old and outdated comments.
**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #
**Special notes for your reviewer**:
**Release note**:
```release-note
NONE
```
@dmmcquay
