pillowfight: run workload in infinite loop by default

Change-Id: I5b5da0f8842632239551de600f351f6849c673be
Reviewed-on: http://review.couchbase.org/41970
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>