netcmd: Add 'samba-tool group stats' command

netcmd: Add 'samba-tool group stats' command

With large domains it's hard to get an idea of how many groups there
are, and how many users are in each group, on average. However, this
could have a big impact on whether a problem can be reproduced or not.

This patch dumps out some summary information so that you can get a
quick idea of how big the groups are.

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Autobuild-User(master): Douglas Bagnall <dbagnall@samba.org>
Autobuild-Date(master): Wed Oct 31 03:40:41 CET 2018 on sn-devel-144