MB-8145: don't expose bucket sasl passwords to read-only admin

Change-Id: I53d65976d1511ce5fcf12dda7ba47c88ab4f593d
Reviewed-on: http://review.couchbase.org/28130
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>