This change adds a new flag `kubeadm token create --print-join-command`. When this flag is passed, kubeadm prints the full `kubeadm join [...]` command, including the CA certificate hash which is otherwise annoying to calculate.

Example:
```
$ kubeadm token create --print-join-command
kubeadm join --token 447067.20b55955bd6abe6c 192.168.99.100:8443 --discovery-token-ca-cert-hash sha256:17023a5c90b996e50c514e63e161e46f78be216fd48c0c3df3be67e008b28889
```