qa: install python3-{cephfs,rados} instead of python34-*

qa: install python3-{cephfs,rados} instead of python34-*

we install the latest python-rpm-macros on all builders since
https://github.com/ceph/ceph-build/pull/1283 . now that we started
building python36-* after that change, for testing the python3 packages on
CentOS/RHEL 7, we need to install python36-* instead of python34-*.

and after the change of 8ae1947, python36-* now "Provides" python3-*, we
can just install python3-* for fulfill the requirement for testing
python3 cephfs bindings.

Fixes: http://tracker.ceph.com/issues/39164
Signed-off-by: Kefu Chai <kchai@redhat.com>
Conflicts: this change is not cherry-picked from master, because,
  in master, we don't install python3 packages after 7e5c85b604.