Introduce bash completions for kubecfg

completions only work for list, get, create, delete.
I'd say this is fairly unusual... Also, I don't really see why you'd want to falltrhough an otherwise empty block. Much more usual would be to group the cases into a single block.

Why not:

```shell
  case ${command} in
    create | list)
      _kubecfg_service_match
      ;;
    update | get | delete)
      _kubecfg_specific_service_match
      ;;
    *)
      ;;
  esac
```