aaa: Fix group population for legacy provider

Legacy provider did not populate groups for added users
in a correct way.

Change-Id: Id1066170758ecddd296b6f253c5bd386cea22008
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1120720
Topic: AAA
Signed-off-by: Yair Zaslavsky <yzaslavs@redhat.com>