As openstack-tox jobs migrated from ubuntu focal to ubuntu
jammy[1]. So horizon integration and selenium-headless jobs
start failing. This patch changes the nodeset for both
these jobs to ubuntu focal as a workaround to unblock
the CI.
[1] https://review.opendev.org/c/openstack/openstack-zuul-jobs/+/861116
Related-Bug: #1996638
Change-Id: I674d7d3a8e1b34229fd5ef0285153e13324d3417