Adds use case to USECASES.md

It is a use case listed in https://wiki.openstack.org/wiki/Puppet-
openstack/ceph-blueprint and it would be a useful example when
considering what the puppet-ceph module can do. It must be linked
from the README.md

Closes-Bug: #1243331
Change-Id: I6d477353a0e1bc00eef77e96099e92845f47f21f