MB-8145: implemented auth for read-only admin

Change-Id: I8eed9d548db82b50e5b1b2b13491cfff7f934ea9
Reviewed-on: http://review.couchbase.org/27952
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>