set the right priorites for the ceph.repo file

Ensures that installing ceph will succeed regardless of other repos having newer ceph packages.

Adds a warning about doing this, we might take it out at a later point but it is critical to release this so installs will work.

This changeset also allows setting the priority in `cephdeploy.conf` and using that.

For output on how this now works and a thorough explanation, see http://tracker.ceph.com/issues/8533