Adding other masters sequentially, not in parallel (#7166)

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

@@ -59,6 +59,7 @@
     --ignore-preflight-errors=all
   register: kubeadm_join_control_plane
   retries: 3
+  throttle: 1
   until: kubeadm_join_control_plane is succeeded
   when:
     - inventory_hostname != groups['kube-master']|first