Upgraded to 1.7.0.
It seems that there are something wrong with the kubeadm v1.7.0, new node can't
join in the cluster in "TLS bootstrap-auto approval" way. So I replace it with
kubeadm v1.6.6.
Because “group auto approver" had been dropped in v1.7, so there is a new strange
task in the master role, It will auto approve any csr every minutes(crontab).
when every node had joined into the cluster, the cronjob will be deleted and finish
the playbook.