If $ceph_repo is set, let's use it when deploying SIG repo.
The feature was only available for Ubuntu until now. But we
could use it for CentOS too.
Thanks to this patch, the Puppet OpenStack CI will now deploy
Ceph repositories from AFS mirrors provided by OpenStack Infra.
Related-Bug: #1676421
Depends-On: Ica91b23c0954ccee17ea13903048b6133758f9f3
Change-Id: I840c0288e5ea15c74578d6468f55acf2456c3326