CI: fix backups Ceph pool name

We configure Cinder to use a 'backups' pool by default, but in CI we
create a 'backup' pool in Ceph. This change fixes CI to use 'backups'.

Change-Id: I53892137eb65ecc9530bd3859c651b12ab228a58
(cherry picked from commit 21efaf3)

Author: markgoddard

roles/cephadm/defaults/main.yml

@@ -17,7 +17,7 @@ cephadm_ceph_osd_devices:
   - secondary2:/dev/cephvg/cephlv
 
 cephadm_ceph_pools:
-  - backup
+  - backups
   - images
   - vms
   - volumes