qa/suites/rados/thrash-old-clients: use cephadm

qa/suites/rados/thrash-old-clients: use cephadm

- deploy cluster with cephadm so we can run a octopus+ cluster and also
  install client packages that are ancient.
- move client.2 back onto the third node, since packages no longer
  conflict.
- test on centos 7.x (i picked 6), since the old releases all built on
  that release.

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