Automatic merge from submit-queue

Add support for running clusters on GCI

Google Container-VM Image (GCI) is the next revision of Container-VM. See documentation at https://cloud.google.com/compute/docs/containers/vm-image/. This change adds support for starting a Kubernetes cluster using GCI.

With this change, users can start a kubernetes cluster using the latest kubelet and kubectl release binary built in the GCI image by running:

	$ KUBE_OS_DISTRIBUTION="gci" cluster/kube-up.sh

Or run a testing cluster on GCI by running:

	$ KUBE_OS_DISTRIBUTION="gci" go run hack/e2e.go -v --up

The commands above will choose the latest GCI image by default.