如何加入节点

Some riffs on this idea:

• A new kubeadm ca-cert-hash command (name TBD) that just prints out the current CA hash. This would basically be the same thing as the openssl command we give in the kubeadm join docs.

• Add a --print-join-command flag for kubeadm token create that prints out the whole join command just like kubeadm init does. This would hopefully make it easy to automate around by running something like kubeadm token create --print-join-command >> worker_init.sh during provisioning.

• A new command kubeadm token get <token id> that gets the full token given the ID, with a --print-join-command flag to optionally print the full join command instead of just the token.