Use "kubeadm join" to join masters to control plane (#6661)

inventory/sample/group_vars/k8s-cluster/k8s-cluster.yml

roles/kubernetes/kubeadm/tasks/main.yml

roles/kubernetes/master/defaults/main/main.yml

roles/kubernetes/master/tasks/kubeadm-secondary-legacy.yml

roles/kubernetes/master/tasks/kubeadm-secondary.yml

roles/kubernetes/master/tasks/kubeadm-setup.yml

roles/kubernetes/preinstall/tasks/0020-verify-settings.yml

roles/kubespray-defaults/defaults/main.yaml

tests/files/packet_centos7-flannel-containerd-addons-ha.yml

tests/files/packet_ubuntu16-flannel-ha.yml

tests/files/packet_ubuntu18-flannel-containerd-ha-once.yml

tests/files/packet_ubuntu18-flannel-containerd-ha.yml