[goal] Migrate freezer-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.

keeping failing jobs running on bionic nodeset till
https://bugs.launchpad.net/freezer/+bug/1896648 is fixed

Depends-On: https://review.opendev.org/#/c/731207/

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

Change-Id: If15af5cccfbc2d1e6e229909f8c1792ddbd7a3c3
Story: #2007865
Task: #40184

.zuul.yaml

@@ -25,6 +25,12 @@
 - job:
     name: freezer-tempest-basic
     parent: devstack-tempest
+    # TODO(gmann): Remove the below nodeset setting to Bionic once
+    # bug https://bugs.launchpad.net/freezer/+bug/1896648 is fixed.
+    # Once nodeset is removed form here then devstack-tempest job
+    # will automatically run this job on Ubuntu Focal nodeset from
+    # Victoria gate onwards.
+    nodeset: openstack-single-node-bionic
     timeout: 7200
     required-projects:
       - openstack/devstack-gate
@@ -115,12 +121,14 @@
 - job:
     name: freezer-tempest-basic-ussuri
     parent: freezer-tempest-basic
+    nodeset: openstack-single-node-bionic
     override-checkout: stable/ussuri
 
 - job:
     name: freezer-tempest-basic-train
     parent: freezer-tempest-basic
     override-checkout: stable/train
+    nodeset: openstack-single-node-bionic
     vars:
       devstack_localrc:
         USE_PYTHON3: True
@@ -128,7 +136,8 @@
 - job:
     name: freezer-tempest-basic-stein
     parent: freezer-tempest-basic
+    nodeset: openstack-single-node-bionic
     override-checkout: stable/stein
     vars:
       devstack_localrc:
-        USE_PYTHON3: True
+        USE_PYTHON3: True