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: I20e9be1071c27301a07089ec70c9ceb53fca7633
diff --git a/.zuul.yaml b/.zuul.yaml
index c46ad4c..31a06dd 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -1,21 +1,25 @@
 - job:
     name: designate-bind9-stable-zed
     parent: designate-bind9
+    nodeset: openstack-single-node-focal
     override-checkout: stable/zed
 
 - job:
     name: designate-bind9-stable-yoga
     parent: designate-bind9
+    nodeset: openstack-single-node-focal
     override-checkout: stable/yoga
 
 - job:
     name: designate-bind9-stable-xena
     parent: designate-bind9
+    nodeset: openstack-single-node-focal
     override-checkout: stable/xena
 
 - job:
     name: designate-bind9-stable-wallaby
     parent: designate-bind9
+    nodeset: openstack-single-node-focal
     override-checkout: stable/wallaby
 
 - project: