MB-8145: do not allow read-only user name to be same as admin

Change-Id: I3711c5a38f3192f02e04c2f49743b1dc42a710aa
Reviewed-on: http://review.couchbase.org/28082
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>