Automatic merge from submit-queue

Register the kubeadm api group in cmd/kubeadm

Fixes the issue mentioned in: https://github.com/kubernetes/kubernetes/pull/34885#issuecomment-253995383

My test config:
```yaml
apiVersion: kubeadm.k8s.io/v1alpha1
kind: MasterConfiguration
kubernetesVersion: v1.4.2
networking:
  podSubnet: 10.244.0.0/16
  dnsDomain: k8s.somecompany.com
  serviceSubnet: 10.16.0.0/12
api:
  externalDNSNames:
   - myawesomek8snode
secrets:
  givenToken: 92f7e2.3685fe2e01f799f3
```
```console
# kubeadm init --config kubeadm.yaml
```
@kubernetes/sig-cluster-lifecycle