[goal] Migrate keystone-tempest-plugin jobs to focal

As per victoria cycle testing runtime and community goal[1]
we need to migrate upstream CI/CD to Ubuntu Focal(20.04).

Most of the Tempest jobs will be migrate automatically once devstack
base job start running on Focal(Depends-On).

Stable jobs testing stable branch needs to keep running on their supported
distro version which is bionic from stein till ussuri.

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

Change-Id: I8c7c12202e5fd024999bb2010bb483b0b2582346
Story: #2007865
Task: #40190
diff --git a/.zuul.yaml b/.zuul.yaml
index b827bce..87f496f 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -19,14 +19,17 @@
 - job:
     name: keystone-dsvm-py3-functional-ussuri
     parent: keystone-dsvm-py3-functional
+    nodeset: openstack-single-node-bionic
     override-checkout: stable/ussuri
 
 - job:
     name: keystone-dsvm-py3-functional-train
     parent: keystone-dsvm-py3-functional
+    nodeset: openstack-single-node-bionic
     override-checkout: stable/train
 
 - job:
     name: keystone-dsvm-py3-functional-stein
     parent: keystone-dsvm-py3-functional
+    nodeset: openstack-single-node-bionic
     override-checkout: stable/stein