Don't require two buckets for testing

This allows tests to be run against an established cluster, rather than
requiring the script to set up the buckets

Also fixed were some issues in automatically inserting the username
if a SASL bucket was being used

Change-Id: I1c81ed5d3718d33c9be1728ba8bbef70dcfbdd12
Reviewed-on: http://review.couchbase.org/25944
Reviewed-by: Rami Chowdhury <rami@esplorio.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>