MB-10415 Make sample loader to use bucket credentials

Change-Id: Ibba52cb2cef7df4fc75986e2381d334fea50196b
Reviewed-on: http://review.couchbase.org/35541
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>