Add Ubuntu 20.04 support and use Python 3 (#6157)

.gitlab-ci/packet.yml

.gitlab-ci/terraform.yml

README.md

Vagrantfile

contrib/terraform/terraform.py

roles/bootstrap-os/molecule/default/molecule.yml

roles/bootstrap-os/tasks/bootstrap-debian.yml

roles/kubernetes/node/vars/ubuntu-20.yml

...fra/image-builder/roles/kubevirt-images/defaults/main.yml

tests/cloud_playbooks/roles/packet-ci/defaults/main.yml

tests/files/packet_ubuntu20-calico-aio.yml