Merge pull request kubernetes#47440 from luxas/kubeadm_stable_v17_version

Automatic merge from submit-queue

kubeadm: Use the v1.7 branch by default

**What this PR does / why we need it**:

Makes kubeadm use the v1.7 branch instead of v1.6

**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #

**Special notes for your reviewer**:
This should be merged right before the rc.0 is cut I guess

**Release note**:

```release-note
NONE
```
@kubernetes/sig-cluster-lifecycle-pr-reviews

Author: Kubernetes Submit Queue

cmd/kubeadm/app/apis/kubeadm/v1alpha1/defaults.go

@@ -26,7 +26,7 @@ import (
 const (
 	DefaultServiceDNSDomain  = "cluster.local"
 	DefaultServicesSubnet    = "10.96.0.0/12"
-	DefaultKubernetesVersion = "stable-1.6"
+	DefaultKubernetesVersion = "stable-1.7"
 	DefaultAPIBindPort       = 6443
 	DefaultDiscoveryBindPort = 9898
 	DefaultAuthorizationMode = "RBAC"