Add tests for bash completion

Given the number of settings that need to be controlled a plain bash
script was a better choice over shelltestrunner. Just a few completions
are attempted, but among them should be the most critical ones (e.g.
“gnt-instance add --node …”.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>