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

Remove configuration variable kubeadm_control_plane

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

@@ -258,12 +258,6 @@
   when: resolvconf_mode is defined
   run_once: true
 
-- name: Stop if kubeadm etcd mode is enabled but experimental control plane is not
-  assert:
-    that: kubeadm_control_plane
-    msg: "kubeadm etcd mode requires experimental control plane"
-  when: etcd_kubeadm_enabled
-
 - name: Stop if etcd deployment type is not host or docker
   assert:
     that: etcd_deployment_type in ['host', 'docker']