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>.

Improve 'kubectl completion` to override short flags

**What this PR does / why we need it**: This PR improves `kubectl completion` to override short flags in
addition to long flags. I tested this PR in both bash and zsh.

```
# Complete pods name in the `default` namespace
$ kubectl get po nginx-4217019353-<tab>
nginx-4217019353-mw1pk  nginx-4217019353-rzw2c

# Complete pods name in the `kube-system` namepace due to overriding the namespace
$ kubectl -n kube-system get po kube<tab>
kube-addon-manager-minikube  kube-dns-910330662-l9pwt     kubernetes-dashboard-9fbhm
```

**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
```