Modify the bench.py example to accept a duration

Run duration is more important than total ops performed

Change-Id: I81290572654a37d1b85b9975c53eb7116dffbb5c
Reviewed-on: http://review.couchbase.org/26389
Reviewed-by: Mordechai Nunberg <mnunberg@haskalah.org>
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>