CEPH_CONF does not exist anymore, resulting both cinder-volume and
cinder-backup being configured with an empty rbd_ceph_conf option.
Using CEPH_CONF_FILE to fix this.
Change-Id: I1aa590aba900a4a94698917e45a0ea5c6f497f18
Signed-off-by: Sébastien Han <seb@redhat.com>