aaa: Change DbUser getters of group fields

The getters will return a copy of the collection
and not an umodifyable collection, as unmofiyable collection
as no default ctor, thus cannot be deserialized by jackson

Topic: AAA
Bug-Url: https://bugzilla.redhat.com/1155084
Change-Id: I03873642ac547b11c66bad42b68c33d7687c8d5a
Signed-off-by: Yair Zaslavsky <yzaslavs@redhat.com>