Increase default quotas for zuul jobs

We're seeing occasional failures in the functional test jobs due
to exceeding volumes quota, so increase these from the default
value of 10 to something more reasonable like 25.

Change-Id: Id1ab94849bfb35256dd0705b888708f3fa1106c4

.zuul.yaml

@@ -11,6 +11,11 @@
       devstack_localrc:
         USE_PYTHON3: true
         VOLUME_BACKING_FILE_SIZE: 16G
+      devstack_local_conf:
+        post-config:
+          $CINDER_CONF:
+            DEFAULT:
+              quota_volumes: 25
 
 - job:
     name: python-cinderclient-functional-py36