qa: disable max_mds changes during thrashing

qa: disable max_mds changes during thrashing

While the trasher supports the behavior desired by issue 10792 [1], the
bugs uncovered due to deactivating MDS (and sometimes killing
deactivating MDS) are presently a distraction from addressing issues
during normal failures. So now thrashing max_mds is turned off by
default. I have added a TODO to deactivate ranks in order (configurably)
as random deactivation causes a lot of other problems.

This also fixes a bug: random.randrange(0.0, 1.0) always returns 0.
Oops.

[1] http://tracker.ceph.com/issues/10792

Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>