This PS is to fix the bug when Workers can't join the cluster (#4276)

because of etc-kubernetes-manifests not empty.

roles/kubernetes/kubeadm/tasks/main.yml

@@ -74,6 +74,7 @@
       command: >-
         {{ bin_dir }}/kubeadm join
         --config {{ kube_config_dir}}/kubeadm-client.conf
+        --ignore-preflight-errors=DirAvailable--etc-kubernetes-manifests
       register: kubeadm_join
       async: 60
       poll: 15