Remove support for CoreOS Container Linux (#6576)

README.md

Vagrantfile

contrib/terraform/aws/README.md

contrib/terraform/aws/variables.tf

contrib/terraform/openstack/README.md

contrib/terraform/packet/README.md

contrib/terraform/terraform.py

contrib/vault/roles/vault/tasks/bootstrap/ca_trust.yml

docs/_sidebar.md

docs/flatcar.md

docs/vagrant.md

roles/bootstrap-os/README.md

roles/bootstrap-os/defaults/main.yml

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

roles/bootstrap-os/tasks/main.yml

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

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

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

roles/download/defaults/main.yml

roles/download/tasks/prep_download.yml