core: miniyum: return group id and not group name

group name returns a description, while groupid returns the group name.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1063941
Change-Id: I6b1b1386c76a6649fa40fa9571755b64f47bb448
Signed-off-by: Alon Bar-Lev <alonbl@redhat.com>