MB-10415 unified access api for admin and ro_admin credentials

... that does not expose passwords

Change-Id: I9ed6c0bfb3aa31f9d7579217dd2910714e1b6ce1
Reviewed-on: http://review.couchbase.org/34788
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>