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: Icc37ad3818d40f17b51827f8b0d19ca0c7c5bade

.zuul.yaml

@@ -20,10 +20,15 @@
       - kuryr-kubernetes-tempest-jobs
     check:
       jobs:
+        - kuryr-kubernetes-tempest-wallaby
         - kuryr-kubernetes-tempest-victoria
         - kuryr-kubernetes-tempest-ussuri
         - kuryr-kubernetes-tempest-train
-        - kuryr-kubernetes-tempest-stein
+
+- job:
+    name: kuryr-kubernetes-tempest-wallaby
+    parent: kuryr-kubernetes-tempest
+    override-checkout: stable/wallaby
 
 - job:
     name: kuryr-kubernetes-tempest-victoria
@@ -48,16 +53,3 @@
       devstack_localrc:
         USE_PYTHON3: True
     voting: false
-
-- job:
-    name: kuryr-kubernetes-tempest-stein
-    parent: kuryr-kubernetes-tempest
-    override-checkout: stable/stein
-    nodeset: openstack-single-node-bionic
-    required-projects:
-    - name: openstack/diskimage-builder
-      override-checkout: 2.30.0
-    vars:
-      devstack_localrc:
-        USE_PYTHON3: True
-    voting: false