When `ceph_stable_rh_storage` is True, every cluster node should have a
`/etc/apt/preferences.d/rhcs.pref` file with the following contents:

```
Explanation: Prefer Red Hat packages
Package: *
Pin: release o=/Red Hat/
Pin-Priority: 999
```

ceph-deploy already did this when used with ice-setup, and we need to do
the same thing with the ceph-ansible stack.

Closes: #1182 and https://bugzilla.redhat.com/show_bug.cgi?id=1404515



Signed-off-by: Sébastien Han <seb@redhat.com>