Add an engine that just returns "ENGINE_NO_BUCKET"

This engine may be installed as the "default" bucket in the
case where a default bucket doesn't exist.

Change-Id: Id775bae2420da4c98f3fd9b9ca2220950f5fee22
Reviewed-on: http://review.couchbase.org/42418
Reviewed-by: Dave Rigby <daver@couchbase.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>