CBD-926: bucket_engine is part of memcached

Change-Id: Ia13d337f55d29089660f4975bf068148db1e8c59
Reviewed-on: http://review.couchbase.org/25509
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>