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>.

 kubeadm: add test coverage to completion.go

**What this PR does / why we need it**:
Add `completion_test.go` with the following tests:
- TestNewCmdCompletion
- TestRunCompletion

A separate commit exports the function GetSupportedShells() to obtain the list of supported shells.

Test coverage is at 96%. The only untested bit is an `io.Writer.Write()` call in `RunCompletion()`. in the case of `bytes.Buffer` it would panic and/or always return `nil` for `error`.

**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
```