We are closing in on adding Ceph Mimic support and
want to ensure we can select either Luminous or Mimic
for running our tests.
For example Ubuntu 18.04 must run Ceph Mimic because
Luminous is not available (yet), Debian doesn't have
Ceph Mimic packages so we must run Luminous there.
Depends-On: https://review.openstack.org/#/c/576247/
Change-Id: If84ecbf86f3ebde8e69a1a26759ad967d061725a
eaacd3f841