Add example usage of plain CNI usage with kubeadm on GCE

[luxas]

In combination with the cloud-controller-manager that handles the cloudy stuff, the networking should be a plain CNI bridge; set by a DaemonSet that drops the following file in /etc/cni/net.d on every node.

Since the kubenet->plain CNI stuff is proceeding, I want to test out how close we are using CNI-only (without the kubenet bells and whistles) on a cloud env like GCE.

I think the CNI spec will look something like this:

{
    "cniVersion": "0.2.0",
    "name": "gcenet",
    "type": "bridge",
    "mtu": 1460,
    "bridge": "cni0",
    "isGateway": true,
    "isDefaultGateway": true,
    "ipMasq": false,
    "hairpinMode": false,
    "ipam": {
        "type": "host-local",
        "subnet": "${bridge-cidr}"
    }
}

(shamelessly inspired from @jbeda's project here: https://github.com/jbeda/kubeadm-gce-tf/blob/master/tf-scripts/prereq.sh#L46 😄)