qa/suites/crimson-rados/thrash: enable chance_down

qa/suites/crimson-rados/thrash: enable chance_down

As the thrash tests were introduced, some options were disabled
until the tests are stabilized.

Re-enable chance_down option (default is 0.4) to detect bugs on restart.

Since it will probably take few iterations before thrash and recovery tests ('default.yaml')
will pass successfully, add anoter 'simple.yaml' which should remain stable.

Fixes: https://tracker.ceph.com/issues/65130
Signed-off-by: Matan Breizman <mbreizma@redhat.com>
(cherry picked from commit 372751789be1725ed06fd077d029295c8cab4c35)