clab and shell completion

[jenslink]

Containerlab has the function to create shell completion but only for the containerlab binary.
/usr/bin/clab is a link to containerlab (at least on my Debian based setup) and it would be relay nice to have shell completion for clab as well.

As a workaround, just replacing containerlab with clab in the output of containerlab completion seems to do the trick.

[jenslink]

And maybe the usage message should also be changed:

_root@clab ~ # clab
deploy container based lab environments with a user-defined interconnections

Usage:
containerlab [command]_