Add stable/wallaby jobs on master gate

We have stable/wallaby released so we should add
their job on master gate to keep branchless tempest
plugins compatible to stable branch.

This also removes the stable/stein job as that is in EM
state now.

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

Change-Id: I9c36e08e8d8e27a97605466953fa79b799fae341

.zuul.yaml

@@ -41,9 +41,13 @@
 - job:
     name: heat-functional-victoria
     parent: heat-functional
-    nodeset: openstack-single-node-bionic
     override-checkout: stable/victoria
 
+- job:
+    name: heat-functional-wallaby
+    parent: heat-functional
+    override-checkout: stable/wallaby
+
 - project:
     templates:
       - check-requirements
@@ -56,6 +60,7 @@
         - heat-functional-orig-train
         - heat-functional-ussuri
         - heat-functional-victoria
+        - heat-functional-wallaby
     gate:
       queue: heat
       jobs: