packages: provide completions out of the box

When installing the binary release from github, of course I need to fix completions my self.
As oh-my-zsh user, I ran 

```bash
stackit completion zsh > ~/.oh-my-zsh/completions/_stackit
omz update
```

But as soon as I use a packaged version (e.g. apt), I'd expect the completions to just work after installation.