[goal] Migrate mistral-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: I3651224de022bff78840df8cd6ca95792dc5cce5
Story: #2007865
Task: #40196

.zuul.yaml

@@ -104,11 +104,13 @@
 - job:
     name: mistral-devstack-ussuri
     parent: mistral-devstack
+    nodeset: openstack-single-node-bionic
     override-checkout: stable/ussuri
 
 - job:
     name: mistral-devstack-train
     parent: mistral-devstack
+    nodeset: openstack-single-node-bionic
     override-checkout: stable/train
     vars:
       devstack_localrc:
@@ -118,6 +120,7 @@
     name: mistral-devstack-stein
     parent: mistral-devstack
     override-checkout: stable/stein
+    nodeset: openstack-single-node-bionic
     vars:
       devstack_localrc:
         USE_PYTHON3: True