Drop support for Fedora 28 and add Fedora 30 and 31 (#5969)

.gitlab-ci/packet.yml

README.md

Vagrantfile

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

roles/container-engine/docker/tasks/main.yml

roles/kubernetes/preinstall/vars/fedora.yml

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

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

tests/files/packet_fedora30-weave.yml

tests/files/packet_fedora31-flannel.yml