Allow ceph backend for nova/cinder/glance

Add a DEVSTACK_GATE_CEPH var that allows a job to specify that it
wants ceph to be used.  When enabled, ceph is used as the backend for
nova, glance, and cinder.

Co-authored-by: Russell Bryant <rbryant@redhat.com>
Change-Id: I27845a8e60ce1a5099152f73f46c451662a6c50b
Merge "Allow ceph backend for nova/cinder/glance"