Automatically infer bootstrap_os (#3498)

Vagrantfile

contrib/terraform/aws/README.md

contrib/terraform/openstack/README.md

docs/coreos.md

inventory/sample/group_vars/all/all.yml

roles/bootstrap-os/tasks/main.yml

roles/kubespray-defaults/defaults/main.yaml

tests/files/do_ubuntu-canal-ha.yml

tests/files/gce_coreos-alpha-weave-ha.yml

tests/files/gce_coreos-calico-aio.yml

tests/files/gce_coreos-canal.yml

tests/files/gce_coreos-cilium.yml

tests/files/gce_coreos-vault-upgrade.yml

tests/files/gce_debian8-calico-upgrade.yml

tests/files/gce_opensuse-canal.yml

tests/files/gce_ubuntu-canal-ha.yml

tests/files/gce_ubuntu-canal-kubeadm.yml

tests/files/gce_ubuntu-flannel-sep.yml

tests/files/gce_ubuntu-rkt-sep.yml

tests/files/gce_ubuntu-vault-sep.yml