schedule_suite: ugly hack to set priority when scheduling

I don't want to add another arg at the end of the very long list of
optional args, so I'll use an environment variable.  Ugly!  But this whole
script needs to be replace anyway.

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #234 from ceph/wip-priority

schedule_suite: ugly hack to set priority when scheduling