Pin stable branch jobs nodeset to Ubuntu Focal (20.04)

In 2023.1 cycle. we are moving the default distro
version of Ubuntu to Jammy (22.04)[1] so we need to pin
the nodeset for stable branch job in master gate so that
they continue run on their supporting distro version which is
Ubuntu Focal since stable/victoria.

[1] https://governance.openstack.org/tc/goals/selected/migrate-ci-jobs-to-ubuntu-jammy.html

Change-Id: If2608dfec183f97727972348b72b0f72a3535b21

.zuul.yaml

@@ -10,11 +10,9 @@
         - vitrage-tempest-plugin-datasources
         - vitrage-tempest-plugin-datasources-yoga
         - vitrage-tempest-plugin-api-yoga
-        - vitrage-tempest-plugin-datasources-xena
-        - vitrage-tempest-plugin-api-xena
-        - vitrage-tempest-plugin-datasources-wallaby:
+        - vitrage-tempest-plugin-datasources-xena:
             voting: false
-        - vitrage-tempest-plugin-api-wallaby
+        - vitrage-tempest-plugin-api-xena
     gate:
       jobs:
         - vitrage-tempest-plugin-api
@@ -232,30 +230,26 @@
 - job:
     name: vitrage-tempest-plugin-api-yoga
     parent: vitrage-tempest-plugin-api
+    nodeset: openstack-single-node-focal
     override-checkout: stable/yoga
 
 - job:
     name: vitrage-tempest-plugin-datasources-yoga
     parent: vitrage-tempest-plugin-datasources
+    nodeset: openstack-single-node-focal
     override-checkout: stable/yoga
 
 - job:
     name: vitrage-tempest-plugin-api-xena
     parent: vitrage-tempest-plugin-api
+    nodeset: openstack-single-node-focal
     override-checkout: stable/xena
 
 - job:
     name: vitrage-tempest-plugin-datasources-xena
     parent: vitrage-tempest-plugin-datasources
+    nodeset: openstack-single-node-focal
     override-checkout: stable/xena
 
-- job:
-    name: vitrage-tempest-plugin-api-wallaby
-    parent: vitrage-tempest-plugin-api
-    override-checkout: stable/wallaby
 
-- job:
-    name: vitrage-tempest-plugin-datasources-wallaby
-    parent: vitrage-tempest-plugin-datasources
-    override-checkout: stable/wallaby