[bp] MB-7381: allow flush with bucket credentials

Change-Id: I535a86fe265dc3bea1881ab4de0b39336c3a0ea1
Original-Reviewed-on: http://review.couchbase.org/23592
Reviewed-on: http://review.couchbase.org/23930
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>