Setting external group as internal group owner adds another level for
access right configuration. Now Administrators can define group in
external system and give it's members possibility to administrate
memberships of Gerrit internal gorups
Change-Id: Idc1874f998314da299c2861840a5131ebccfdd78
Signed-off-by: Dariusz Luksza <dariusz@luksza.org>