Support MasterConfiguration

Like this

```yaml
kind: MasterConfiguration
apiVersion: kubeadm.k8s.io/v1alpha1
controllerManagerExtraArgs:
  horizontal-pod-autoscaler-use-rest-clients: "true"
  horizontal-pod-autoscaler-sync-period: "10s"
  node-monitor-grace-period: "10s"
apiServerExtraArgs:
  runtime-config: "api/all=true"
kubernetesVersion: "stable-1.8"
```