Automatic merge from submit-queue
kubeadm: refactor discovery behind an interface
This adds support for alternative discovery methods using discovery urls. It is a breaking change. This is a WIP.
Example usage:
```
$ kubeadm init --discovery token://
$ kubeadm join --discovery token://c05de9:ab224260fb3cd718@192.168.0.1:6555,191.168.0.2:6443
$ kubeadm join --discovery file:///etc/kubernetes/cluster.json
$ kubeadm join --discovery https://storage.google.apis.com/kube-discovery/98ea6e4/kubeconfig.json
```
@kubernetes/sig-cluster-lifecycle
8d71970dcd