Automatic merge from submit-queue
Support completion for --cluster and --user
**What this PR does / why we need it**:
kubectl is supported completion for `--cluster` and `--user`.
```
$ kubectl --user=<tab>
--user=minikube --user=user01
$ kubectl --cluster=<tab>
--cluster=cluster01 --cluster=minikube
```
**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
Add completion support for --namespace and --cluster to kubectl
```
c3463d737e