Introduce bash completions for kubecfg

completions only work for list, get, create, delete.
So if you use __contains_word on a non-empty list of words then the exit status will be the failure of `[[ $w = "$word" ]]` which will be 1. However, if you use it on an empty list, then exit status will be 0 and I think it shouldn't...

```shell
$ __contains_word x 1 2 3
$ echo $?
1
$ __contains_word x
$ echo $?
0
```

I think the `return 1` is needed here.

I noticed the __contains_word in systemd has this bug, I'll try to send them a patch to correct that there.