qa/suites/rados/thrash: force normal pg log length with cache tiering

qa/suites/rados/thrash: force normal pg log length with cache tiering

When we are doing cache tiering, we are more sensitive to short PG logs
because the dup op entries are not perfectly promoted from the base to
the cache.

See:
 http://tracker.ceph.com/issues/38358
 http://tracker.ceph.com/issues/24320

This works around the problem by not testing short pg logs in combination
with cache tiering.  This works because the short_pg_log.yaml fragment
sets the short log in the [global] section but the cache workloads overload
it (back to a large/default value) in the [osd] section.

Signed-off-by: Sage Weil <sage@redhat.com>