证书过期了会提示：
Unable to connect to the server: x509: certificate has expired or is not yet valid
查看证书过期时间：
kubeadm certs check-expiration kubeadm alpha certs check-expiration # 1.18之前版本
轮换证书：
kubeadm certs renew all kubeadm alpha certs renew all
kubectl用到的配置文件：
cp /etc/kubernetes/admin.conf /root/.kube/config
根据提示逐个节点重启kubelet、controller-manager、scheduler
systemctl restart kubelet.service