Update stable branch jobs on master gate

We have stable/xena to stable/zed are the supported
stable branches and we should add their job on master gate
to keep branchless tempest plugins compatible to those branch.

This also removes the old EM stable branch jobs.

Ref: Tempest plugins guide for stable branch testing:
- https://docs.openstack.org/tempest/latest/stable_branch_testing_policy.html

Change-Id: Ie8278999af25be7e7d88304dca6b3b5ad24a1c3d

.zuul.yaml

@@ -89,7 +89,9 @@
             branches: ^(?!stable/(newton|ocata)).*$
         - mistral-devstack-kombu:
             branches: ^(?!stable/newton).*$
-        - mistral-devstack-ussuri
+        - mistral-devstack-zed
+        - mistral-devstack-yoga
+        - mistral-devstack-xena
     queue: mistral
     gate:
       jobs:
@@ -100,7 +102,18 @@
         - mistral-devstack-kombu
 
 - job:
-    name: mistral-devstack-ussuri
+    name: mistral-devstack-zed
     parent: mistral-devstack
-    nodeset: openstack-single-node-bionic
-    override-checkout: stable/ussuri
+    nodeset: openstack-single-node-focal
+    override-checkout: stable/zed
+- job:
+    name: mistral-devstack-yoga
+    parent: mistral-devstack
+    nodeset: openstack-single-node-focal
+    override-checkout: stable/yoga
+
+- job:
+    name: mistral-devstack-xena
+    parent: mistral-devstack
+    nodeset: openstack-single-node-focal
+    override-checkout: stable/xena