As per victoria cycle testing runtime and community goal[1]
we need to migrate upstream CI/CD to Ubuntu Focal(20.04).
devstack-plugin-nfs jobs will be migrate automatically to Focal
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: Ia65c0739f9e8454e7bc8ada7cc076f42c68fe7ce