Use bucket name in SASL if username omitted

Change-Id: Idd4837b33dfb63b7834436e6a9d6f5302ce3d31c
Reviewed-on: http://review.couchbase.org/28532
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>