ci: re-add osd_pool_default_size to 1 with the override

If we don't do this the client will create pools with a replica 3 since
osd_pool_default_size was gone in ceph-override.json. This was making
switch_to_containers failing.

Signed-off-by: Sébastien Han <seb@redhat.com>

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

@@ -1,7 +1,8 @@
 {
 	"ceph_conf_overrides": {
 		"global": {
-			"osd_pool_default_pg_num": 12
+			"osd_pool_default_pg_num": 12,
+			"osd_pool_default_size": 1
 		}
 	}
 }