[goal] Migrate telemetry-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.

Depends-On: https://review.opendev.org/#/c/752294/

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

Change-Id: I7e032bb8848dd662432f846c51989180aa7c9b28
Story: #2007865
Task: #40223
diff --git a/.zuul.yaml b/.zuul.yaml
index a5bb820..3cbb1a5 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -130,11 +130,13 @@
 - job:
     name: telemetry-dsvm-integration-ussuri
     parent: telemetry-dsvm-integration
+    nodeset: openstack-single-node-bionic
     override-checkout: stable/ussuri
 
 - job:
     name: telemetry-dsvm-integration-train
     parent: telemetry-dsvm-integration
+    nodeset: openstack-single-node-bionic
     override-checkout: stable/train
     vars:
       devstack_localrc: