Remove rkt support (#4671)

.gitlab-ci/gce.yml

.gitlab-ci/packet.yml

README.md

contrib/vault/roles/vault/tasks/shared/check_vault.yml

contrib/vault/roles/vault/templates/rkt.service.j2

contrib/vault/vault.md

docs/ansible.md

docs/roadmap.md

docs/vars.md

roles/container-engine/meta/main.yml

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

roles/container-engine/rkt/files/rkt-gc.sh

roles/container-engine/rkt/tasks/install.yml

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

roles/container-engine/rkt/vars/debian.yml

roles/container-engine/rkt/vars/fedora.yml

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

roles/container-engine/rkt/vars/suse.yml

roles/download/templates/kubeadm-images.yaml.j2

roles/etcd/tasks/install_rkt.yml