TripleO Stein to Deploy Ceph Mimic or Nautilus

TripleO Rocky has been tested to deploy Ceph Lumionus. During the Stein cycle we should start to develop against a newer version of Ceph. Mimic was released June 2018 and early versions of Nautilus should be available before the predicted Stein feature freeze. As in Pike and Rocky Ceph will only be deployed in containers.

New Ceph containers can be easily tested in the existing TripleO CI by submitting a WIP patch which overrides the default DockerCephDaemonImage with a URL to a newer version of the ceph container daemon, e.g. https://review.openstack.org/#/c/562213 . If the relevant tests pass the default may be updated but still easily rolled back. During the Stein cycle we should test both the Mimic and Nautilus containers provided by the Ceph team the same way and decide on a stable default version.