qa: downgrade librados2,librbd1 for thrash-old-clients tests

qa: downgrade librados2,librbd1 for thrash-old-clients tests

librados2 and librbd1 are installed as a dependency of qemu-kvm.
qemu-kvm is installed by ceph-cm-ansible, see [1].

in thrash-old-clients, jewel packages are installed, but yum does
not allow downgrade unless it's required explicitly. in this change,
we downgrade librbd1 and librados2 to address this issue.

currently, the ceph packages shipped by CentOS/RHEL 7 are still an old
version of jewel. so this issue only kicks in when we try to install
hammer.

this change should address failures like

Command failed on smithi136 with status 1: '\n sudo yum -y install
rbd-fuse\n '

found in rados/thrash-old-clients tests.

---
[1]
https://github.com/ceph/ceph-cm-ansible/commit/3db1cbdc226ecdea532bcc7cd6e710b5ad2bb0d6#diff-f2b05d775fedff6c5c6689f564b32f1c

Fixes: http://tracker.ceph.com/issues/37618
Signed-off-by: Kefu Chai <kchai@redhat.com>