cbc: Allow to force SASL authentication mechanism

Change-Id: Icf06230f900ed9246e2905eedd60a708580d2ce8
Reviewed-on: http://review.couchbase.org/30251
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Mark Nunberg <mnunberg@haskalah.org>