Fix instance shutdown automatic worker deletion on clouds

* Fix a regression caused by lowering the Kubelet TLS client
certificate to system:nodes group (#100) since dropping
cluster-admin dropped the Kubelet's ability to delete nodes.
* On clouds where workers can scale down (manual terraform apply,
AWS spot termination, Azure low priority deletion), worker shutdown
runs the delete-node.service to remove a node to prevent NotReady
nodes from accumulating
* Allow Kubelets to delete cluster nodes via system:nodes group. Kubelets
acting with system:node and kubelet-delete ClusterRoles is still an
improvement over acting as cluster-admin

google-cloud/fedora-atomic/kubernetes/bootkube.tf

@@ -1,6 +1,6 @@
 # Self-hosted Kubernetes assets (kubeconfig, manifests)
 module "bootkube" {
-  source = "git::https://github.com/poseidon/terraform-render-bootkube.git?ref=bcbdddd8d07c99ab88b2e9ebfb662de4c104de0a"
+  source = "git::https://github.com/poseidon/terraform-render-bootkube.git?ref=e892e291b572655699aee8565c14c8446bab2104"
 
   cluster_name          = "${var.cluster_name}"
   api_servers           = ["${format("%s.%s", var.cluster_name, var.dns_zone)}"]