Fix Prometheus etcd scrape config for DigitalOcean

* Kubelet uses a node's hostname as the node name, which isn't
resolvable on DigitalOcean. On DigitalOcean, the node name was
set to the internal IP until #337 switched to instead configuring
kube-apiserver to prefer the InternalIP for communication
* Explicitly configure etcd scrapes to target each controller by
internal IP and port 2381 (replace __address__)

addons/prometheus/config.yaml

@@ -102,7 +102,7 @@ data:
           regex: 'true'
         - action: labelmap
           regex: __meta_kubernetes_node_label_(.+)
-        - source_labels: [__meta_kubernetes_node_name]
+        - source_labels: [__meta_kubernetes_node_address_InternalIP]
           action: replace
           target_label: __address__
           replacement: '${1}:2381'