Fix Azure delete-node script to lowercase hostnames

* Fix issue where worker nodes didn't delete themselves on
scale-down or deallocation (e.g. low priority instances).
Lowercase the hostname and delete the Kubernetes node
* Kubelet registers the lowercase hostname as the node name,
but Azure workers get hostname CLUSTER-worker-GENERATED where
the generated identifier may contain uppercase characters

azure/container-linux/kubernetes/workers/cl/worker.yaml.tmpl

@@ -113,7 +113,7 @@ storage:
             docker://k8s.gcr.io/hyperkube:v1.11.2 \
             --net=host \
             --dns=host \
-            --exec=/kubectl -- --kubeconfig=/etc/kubernetes/kubeconfig delete node $(hostname)
+            --exec=/kubectl -- --kubeconfig=/etc/kubernetes/kubeconfig delete node $(hostname | tr '[:upper:]' '[:lower:]')
 passwd:
   users:
     - name: core