Correct "Increase default quotas for zuul jobs"

It turns out that you can't use the devstack post_config phase
for this because it happens too late.  The quota settings must
be made to cinder.conf before cinder-manage db_sync is called
where they're used to set the max limit for the tables used by
the db quotas driver.

Depends-on: https://review.opendev.org/c/openstack/devstack/+/803521
Change-Id: I3bb7d9f9bede063573b24fb5a33a364c53d52434

.zuul.yaml

@@ -11,11 +11,9 @@
       devstack_localrc:
         USE_PYTHON3: true
         VOLUME_BACKING_FILE_SIZE: 16G
-      devstack_local_conf:
-        post-config:
-          $CINDER_CONF:
-            DEFAULT:
-              quota_volumes: 25
+        CINDER_QUOTA_VOLUMES: 25
+        CINDER_QUOTA_BACKUPS: 25
+        CINDER_QUOTA_SNAPSHOTS: 25
     irrelevant-files:
       - ^.*\.rst$
       - ^doc/.*$