Description of problem:
When installing Ceph on Ubuntu with ceph-ansible, we are trusting that our package versions in Red Hat Ceph Storage will always be newer than the packages in Ubuntu itself.

The implication here is that if the Ubuntu maintainer ever ships a newer package than what we ship, then Ubuntu's package will override ours.

With ceph-deploy and ice-setup, we used to use apt pinning to make Ubuntu nodes prefer Red Hat's packages. We need ceph-ansible to do the same.

Version-Release number of selected component (if applicable):
all versions of ceph-ansible prior to 2.1

How reproducible:
always

Steps to Reproduce:
1. Use ceph-ansible to set up a cluster
2. Look in /etc/apt/preferences.d on each cluster node

Actual results:
No Red Hat pref file exists

Expected results:
An rhcs.pref file exists, and "ceph --version" prints something with "redhat" in it.