Add RHEL support subscription registration (#6572)

Vagrantfile

docs/centos8.md

docs/offline-environment.md

docs/rhel.md

inventory/sample/group_vars/all/all.yml

roles/bootstrap-os/handlers/main.yml

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

roles/bootstrap-os/tasks/main.yml

roles/container-engine/containerd/defaults/main.yml

roles/container-engine/containerd/tasks/containerd_repo.yml

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

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

roles/container-engine/docker/vars/redhat.yml