Add ramp_time option

Add ramp_time option

Sometimes it's useful to let a job settle for a little while
before taking any measurements on latency and throughput, since
the initial rate on eg a write workload may be much higher than
the longer sustained rate.

So add a ramp_time option that allows the user to specify a lead
ramp time that must have passed before fio takes any performance
numbers into account.

Suggested by "Jenkins, Lee" <Lee.Jenkins@hp.com>

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>