tests: set pool size to 1 in ceph-override.json

setting this setting to 1 makes the CI covering the related code in the
playbook without breaking the upgrade scenarios.

Those scenarios were broken because there is a check `TASK [waiting for
clean pgs...]` in rolling_update.yml, since the pool size for
`cephfs_metadata` and `cephfs_data` are updated to `2` in
`ceph-override.json` and there is not enough osd to honor this size,
some PGs are degraded and make the mentioned check failing.

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>

tests/functional/centos/7/cluster/ceph-override.json

@@ -8,12 +8,12 @@
 		{
 		  "name": "cephfs_metadata",
 		  "pgs": 8,
-		  "size": 2
+		  "size": 1
 		},
 		{
 		  "name": "cephfs_data",
 		  "pgs": 8,
-		  "size": 2
+		  "size": 1
 		}
 	],
   "ceph_mon_docker_memory_limit": "2g"