core: Fix admin@internal login

This fixes InternalBrokerUtils to query for
users not only by user name but also domain.
This will allow proper login in case there are
several users with the same name, but in
different domains

Bug-Url: https://bugzilla.redhat.com/1095420
Change-Id: Ifb2455380242a7b409dd8ea0376e049127996d09
Signed-off-by: Yair Zaslavsky <yzaslavs@redhat.com>