New e2e job to test Kubernetes 1.6 using kubeadm 1.7.
New e2e job to test Kubernetes 1.6 using kubeadm 1.7.

Since kubeadm 1.7 supports installing the 1.6 control plane, create an
e2e job to exercise it.

Fixes https://github.com/kubernetes/kubeadm/issues/301.

Adds the job to these testgrid tabs:
  - google-gce
  - release-1.7-all
  - release-1.7-blocking

GoogleCloudPlatform/testgrid,GoogleCloudPlatform/testgrid,GoogleCloudPlatform/testgrid,GoogleCloudPlatform/testgrid,GoogleCloudPlatform/testgrid,GoogleCloudPlatform/testgrid