Add stable/zed jobs on master gate

As zed is released, we should add its job on master
gate to keep branchless tempest plugins compatible
to stable branch.

Also fixing the zuul config error.

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

Change-Id: Ie4e6f5eca6b91e78953fad6c592e21c5d5f8608b

.zuul.yaml

@@ -1,4 +1,5 @@
 - project:
+    queue: solum
     templates:
       - check-requirements
       - tempest-plugin-jobs
@@ -7,8 +8,12 @@
         - solum-devstack-yoga
         - solum-devstack-xena
         - solum-devstack-wallaby
-    gate:
-      queue: solum
+
+- job:
+    name: solum-devstack-zed
+    parent: solum-devstack
+    nodeset: openstack-single-node-focal
+    override-checkout: stable/zed
 
 - job:
     name: solum-devstack-yoga