Test ansible-devel with an ubuntu-focal bridge.o.o

As of https://github.com/ansible/ansible/commit/724800c (and now
2.12.0b1), ansible started requiring Python 3.8 or later on
controllers. Switch our representative bridge.openstack.org test
nodes to the ubuntu-focal label which has 3.8.10 as its default
python3 so we can determine whether it's safe to upgrade production
similarly.

Change-Id: Ie1dc4dfaaf08ab74bf59717610231855926e9d19

zuul.d/system-config-run.yaml

@@ -75,6 +75,16 @@
     parent: system-config-run-base
     description: |
       Run the base playbook with the latest ansible.
+    nodeset:
+      nodes:
+        - name: bridge.openstack.org
+          label: ubuntu-focal
+        - name: xenial
+          label: ubuntu-xenial
+        - name: bionic
+          label: ubuntu-bionic
+        - name: focal
+          label: ubuntu-focal
     required-projects:
       - name: github.com/ansible/ansible
         override-checkout: devel