[goal] Migrate vitrage-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: Ia69f906364d6fe5a31b14dc8ec9cd37e18bdf27d
Story: #2007865
Task: #40226

.zuul.yaml

@@ -22,11 +22,13 @@
 - job:
     name: vitrage-dsvm-api-py3-ussuri
     parent: vitrage-dsvm-api-py3
+    nodeset: openstack-single-node-bionic
     override-checkout: stable/ussuri
 
 - job:
     name: vitrage-dsvm-api-py3-train
     parent: vitrage-dsvm-api-py3
+    nodeset: openstack-single-node-bionic
     override-checkout: stable/train
     vars:
       devstack_localrc:
@@ -35,6 +37,7 @@
 - job:
     name: vitrage-dsvm-api-py3-stein
     parent: vitrage-dsvm-api-py35
+    nodeset: openstack-single-node-bionic
     override-checkout: stable/stein
     vars:
       devstack_localrc:
@@ -43,11 +46,13 @@
 - job:
     name: vitrage-dsvm-datasources-py3-ussuri
     parent: vitrage-dsvm-datasources-py3
+    nodeset: openstack-single-node-bionic
     override-checkout: stable/ussuri
 
 - job:
     name: vitrage-dsvm-datasources-py3-train
     parent: vitrage-dsvm-datasources-py3
+    nodeset: openstack-single-node-bionic
     override-checkout: stable/train
     vars:
       devstack_localrc:
@@ -56,7 +61,8 @@
 - job:
     name: vitrage-dsvm-datasources-py3-stein
     parent: vitrage-dsvm-datasources-py35
+    nodeset: openstack-single-node-bionic
     override-checkout: stable/stein
     vars:
       devstack_localrc:
-        USE_PYTHON3: True
+        USE_PYTHON3: True