Add cpus_allowed option

cpumask can be cumbersome to use, since it requires you to calculate
a decimal mask to pass to fio. So add a cpus_allowed option that
takes a text list of allowed cpus, ala: cpus_allowed=1,3,8 will set
the affinity mask for CPUS 1, 3, and 8.

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