aaa: Fix group name issues for legacy provider

Chagning format from -  authz/a/b/c/d to -  a/b/c/d@authz
to conform to the users format.
Removed specific code at rest-api that handled old format

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1120720
Change-Id: I8c1feed08e33156c794b230fa04f33f2ebd82240
Topic: AAA
Signed-off-by: Yair Zaslavsky <yzaslavs@redhat.com>