Automatic merge from submit-queue
kubeadm: Default to using token discovery.
Recent changes to support multiple methods for discovery meant that
"kubeadm init" no longer was sufficient and users would need to add
"--discovery token://" to achieve the same results.
Instead lets assume discovery if the user does not specify anything else
to maintain parity and the brevity of our original instructions.
**Release note**:
```release-note
NONE
```
CC @mikedanese @luxas
58d319e589