Add gevent (iops) option for bench.py

Change-Id: I5df5d8a4c85981298f1741bcf34050bec3816c2d
Reviewed-on: http://review.couchbase.org/29060
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Volker Mische <volker.mische@gmail.com>