This introduces a new -g/--group argument to filter containers based on
their groups.

This supports the rather obvious: --group blah
Which will only list containers that are in group blah.

It may also be passed multiple times: --group blah --group bleh
Which will list containers that are in either (or both) blah or bleh.

And it also takes: --group blah,bleh --group doh
Which will list containers that are either in BOTH blah and bleh or in doh.
Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Acked-by: Michael H. Warfield <mhw@WittsEnd.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>