Automatic merge from submit-queue (batch tested with PRs 61601, 62881, 63159). 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: add test coverage to join.go

**What this PR does / why we need it**:
Add test coverage to `join.go`.

A separate commit exports the function `ValidateJoinCommandLine()` from `join.go` so that testing this file is more flexible.

Test coverage is at 76%. One untested part is successfully running `Join.Run()` without errors, but that requires a valid HTTPS API server running and a valid config. i got this partially working but gave up because i faced some cert / config blockers. suggestions on how to get that to work easily are welcome.

**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:

NONE

**Special notes for your reviewer**:

NONE

**Release note**:

```release-note
NONE
```