Add stable ussuri job on master gate

stable/ussuri branch is setup now
- https://review.opendev.org/#/q/topic:qa-ussuri-release+status:merged+projects:openstack/devstack

and Tempest and plugins master version is used to test the stable/ussuri.

Adding a stable job for ussuri on master so that any new tests
or changes does not break the stable ussuri testing.

Tempest already added the stable/ussuri job on master gate so adding it
for plugins is safe now.
- https://review.opendev.org/#/c/723620/

Change-Id: If8afdbd9b933bf1b07bef1dec54f54a46917d9fd

.zuul.yaml

@@ -28,6 +28,11 @@
       branch_override: stable/train
       use_python3: 1
 
+- job:
+    name: heat-functional-ussuri
+    parent: heat-functional
+    override-checkout: stable/ussuri
+
 - project:
     templates:
       - check-requirements
@@ -41,6 +46,7 @@
         - heat-functional-orig-stein
         - heat-functional-convg-train
         - heat-functional-orig-train
+        - heat-functional-ussuri
     gate:
       queue: heat
       jobs: