aaa: use Authn.Capabilities.AUTHENTICATE_CREDENTIALS correctly

Topic: AAA
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1120720
Change-Id: I8ed1d4523ddf2bcefb3f1b1be868e4caf6c04ad7
Signed-off-by: Alon Bar-Lev <alonbl@redhat.com>