[goal] Migrate cinder-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[2].

Stable jobs testing stable branch needs to keep running on their supported
distro version which is bionic from stein till ussuri.

Keeping barbican jobs running on bionic nodeset till
https://storyboard.openstack.org/#!/story/2007732 is fixed.

[1] https://governance.openstack.org/tc/goals/selected/victoria/migrate-ci-cd-jobs-to-ubuntu-focal.html
[2] https://review.opendev.org/#/c/734700/

Story: #2007865
Task: #40179

Change-Id: Ief046344957004a5ad29337864ace5837d6c553d
1 file changed