aaa: Fix "directory group" sub tab

Some changes were made to the directory group sub tab
of users:

1. Organizational unit is removed, as relevant for ldap providers, and
engine/ui should be agnostic to provider type

2. namespace is introduced , same as in other places in ui
(for example at the add user/group dialog).

Topic: AAA
Change-Id: I2125c11f60576d9c2fd9fa3d2771cb34145cccc2
Signed-off-by: Yair Zaslavsky <yzaslavs@redhat.com>