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: I5f4f1338a60634af8ae062475e314feb55c4a26d
diff --git a/.zuul.yaml b/.zuul.yaml
index 45ccd43..29a7ee2 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -1,16 +1,19 @@
 - job:
     name: heat-functional-yoga
     parent: heat-functional
+    nodeset: openstack-single-node-focal
     override-checkout: stable/yoga
 
 - job:
     name: heat-functional-xena
     parent: heat-functional
+    nodeset: openstack-single-node-focal
     override-checkout: stable/xena
 
 - job:
     name: heat-functional-wallaby
     parent: heat-functional
+    nodeset: openstack-single-node-focal
     override-checkout: stable/wallaby
 
 - project: