Don't allow access to default bucket with password only

Change-Id: I2af3e34e88ba63302473917362682b987a071f9e
Reviewed-on: http://review.couchbase.org/27759
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>