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: I4b8747d2ce5f73a467678883bbc71d388ea8f228
Story: #2007865
Task: #40183