aaa: Fix UserGroups list model

The previous code populated the groups sub tab of users
by using information from the user entity (namespace and
group names), but the namespace may vary between user
and the groups it belongs to

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1120720
Change-Id: I39387d68687d530008a2114057d30fc04f27f0ce
Signed-off-by: Yair Zaslavsky <yzaslavs@redhat.com>