aaa: add authn negotiation priority

Topic: AAA
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1120720
Change-Id: I1bb8803e44fbc75ff91c0b6791491ced58eff6d5
Signed-off-by: Alon Bar-Lev <alonbl@redhat.com>