MB-100: add active_tasks to buildout config

Change-Id: Iba728880f66021024b3878ea4333a6f4d66cb1d6
Reviewed-on: http://review.couchbase.org/24415
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>
Tested-by: Pavel Paulau <pavel.paulau@gmail.com>