Automatic merge from submit-queue
Move shell completion generation into 'kubectl completion' command
Remove static shell completion scripts from the repo and add `completion` command to kubectl:
```bash
$ source <(kubectl completion bash)
```
or
```bash
$ source <(kubectl completion zsh)
```
This makes maintenance easier because no static scripts must be generated and committed anymore in the repo.
Moreover, kubectl is self-contained again for the user including the latest completion code. I am thinking about the use-case of updating kubectl via gcloud (or some package manager). The completion code is always in-sync, without the need to download a `contrib/completion/bash/kubectl` file from github.
Opinions are welcome /cc @eparis @nak3
Fixes https://github.com/openshift/origin/issues/5290
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