In usecases where only cinder is using ceph we currently do not
execute ceph_client role, which makes nodes failing to spawn instances
from RBD volumes.
Sample usecase where Glance might be using Swift and it might be desired to use
local storage for Nova ephemeral drives, but cinder spawning volumes
on Ceph
Currently this can be workarounded with setting `nova_rbd_inuse: True` but
at the same time `nova_libvirt_images_rbd_pool: ''`, though this is
counter-intuitive and this patch aims to improve this.
Change-Id: I412d1e9ccb51f0cd33a98333bfa1a01510867fbe
5300fcea9d