[goal] Migrate zun-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/744596/

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

Change-Id: I30ffa2328a5739e229a06729d471e31b9be53be5
Story: #2007865
Task: #40230

.zuul.yaml

@@ -1,6 +1,7 @@
 - job:
     name: zun-tempest-py35-docker-sql-rocky
     parent: zun-tempest-py35-docker-sql
+    nodeset: openstack-single-node-xenial
     override-checkout: stable/rocky
     vars:
       branch_override: stable/rocky
@@ -8,6 +9,7 @@
 - job:
     name: zun-tempest-py35-docker-sql-stein
     parent: zun-tempest-py35-docker-sql
+    nodeset: openstack-single-node-bionic
     override-checkout: stable/stein
     vars:
       branch_override: stable/stein
@@ -15,6 +17,7 @@
 - job:
     name: zun-tempest-py3-docker-sql-train
     parent: zun-tempest-py3-docker-sql
+    nodeset: openstack-single-node-bionic
     override-checkout: stable/train
     vars:
       branch_override: stable/train
@@ -22,6 +25,7 @@
 - job:
     name: zun-tempest-py3-docker-sql-ussuri
     parent: zun-tempest-py3-docker-sql
+    nodeset: openstack-single-node-bionic
     override-checkout: stable/ussuri
 
 - project: