pillowfight: reclarify set/get rates

The --ratio option has been renamed to --set-pct (with the alias `-r`
still being preserved).

Change-Id: I729bfe3eb88452640fe90955ca1fd28b541a0e8b
Reviewed-on: http://review.couchbase.org/41362
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>