Automatic merge from submit-queue

kubeadm: init must validate or generate token before anything else.

**What this PR does / why we need it**: `kubeadm init` must validate or generate a token before anything else. Otherwise, if token validation or generation fail, one will need to run `kubeadm reset && systemctl restart kubelet` before re-running `kubeadm init`.

**Which issue this PR fixes**: fixes kubernetes/kubeadm#112

**Special notes for your reviewer**: /cc @luxas

Tested manually.

### With no token

```
$ sudo ./kubeadm init
[kubeadm] WARNING: kubeadm is in alpha, please do not use it for production clusters.
[preflight] Running pre-flight checks
[init] Using Kubernetes version: v1.5.2
[token-discovery] A token has not been provided, generating one
[certificates] Generated Certificate Authority key and certificate.
[certificates] Generated API Server key and certificate
[certificates] Generated Service Account signing keys
[certificates] Created keys and certificates in "/etc/kubernetes/pki"
[kubeconfig] Wrote KubeConfig file to disk: "/etc/kubernetes/admin.conf"
[kubeconfig] Wrote KubeConfig file to disk: "/etc/kubernetes/kubelet.conf"
[apiclient] Created API client, waiting for the control plane to become ready
[apiclient] All control plane components are healthy after 7.762803 seconds
[apiclient] Waiting for at least one node to register and become ready
[apiclient] First node is ready after 1.003148 seconds
[apiclient] Creating a test deployment
[apiclient] Test deployment succeeded
[token-discovery] Using token: 8321b6:a535ba541af7623c
[token-discovery] Created the kube-discovery deployment, waiting for it to become ready
[token-discovery] kube-discovery is ready after 1.003423 seconds
[addons] Created essential addon: kube-proxy
[addons] Created essential addon: kube-dns
Your Kubernetes master has initialized successfully!
You should now deploy a pod network to the cluster.
Run "kubectl apply -f [podnetwork].yaml" with one of the options listed at:
    http://kubernetes.io/docs/admin/addons/
You can now join any number of machines by running the following on each node:
kubeadm join --discovery token://8321b6:a535ba541af7623c@10.142.0.6:9898
```

### With invalid token

```
$ sudo ./kubeadm init --discovery token://12345:12345
[kubeadm] WARNING: kubeadm is in alpha, please do not use it for production clusters.
[preflight] Running pre-flight checks
[init] Using Kubernetes version: v1.5.2
[token-discovery] A token has been provided, validating [&{ID:12345 Secret:12345 Addresses:[]}]
token ["12345:12345"] was not of form ["^([a-z0-9]{6})\\:([a-z0-9]{16})$"]
```

### With valid token

```
$ sudo ./kubeadm ex token generate
cd540e:c0e0318e2f4a63b1

$ sudo ./kubeadm init --discovery token://cd540e:c0e0318e2f4a63b1
[kubeadm] WARNING: kubeadm is in alpha, please do not use it for production clusters.
[preflight] Running pre-flight checks
[init] Using Kubernetes version: v1.5.2
[token-discovery] A token has been provided, validating [&{ID:cd540e Secret:c0e0318e2f4a63b1 Addresses:[]}]
[certificates] Generated Certificate Authority key and certificate.
[certificates] Generated API Server key and certificate
[certificates] Generated Service Account signing keys
[certificates] Created keys and certificates in "/etc/kubernetes/pki"
[kubeconfig] Wrote KubeConfig file to disk: "/etc/kubernetes/admin.conf"
[kubeconfig] Wrote KubeConfig file to disk: "/etc/kubernetes/kubelet.conf"
[apiclient] Created API client, waiting for the control plane to become ready
[apiclient] All control plane components are healthy after 13.513305 seconds
[apiclient] Waiting for at least one node to register and become ready
[apiclient] First node is ready after 0.502656 seconds
[apiclient] Creating a test deployment
[apiclient] Test deployment succeeded
[token-discovery] Using token: cd540e:c0e0318e2f4a63b1
[token-discovery] Created the kube-discovery deployment, waiting for it to become ready
[token-discovery] kube-discovery is ready after 2.002457 seconds
[addons] Created essential addon: kube-proxy
[addons] Created essential addon: kube-dns
Your Kubernetes master has initialized successfully!
You should now deploy a pod network to the cluster.
Run "kubectl apply -f [podnetwork].yaml" with one of the options listed at:
    http://kubernetes.io/docs/admin/addons/
You can now join any number of machines by running the following on each node:
kubeadm join --discovery token://cd540e:c0e0318e2f4a63b1@10.142.0.6:9898
```

**Release note**:
```release-note
NONE
```