coreos/azure: Use drop-in to fix the interwebz -

It turns out `network-online.target` doesn't imply that there is
access to the Internet and simply using `curl --retry 10` doesn't
work and hangs forever, even if we can already reach github.com...

docs/getting-started-guides/coreos/azure/cloud_config_templates/kubernetes-cluster-main-nodes-template.yml

@@ -56,6 +56,14 @@ coreos:
     group: stable
     reboot-strategy: off
   units:
+    - name: systemd-networkd-wait-online.service
+      drop-ins:
+        - name: 50-check-github-is-reachable.conf
+          content: |
+            [Service]
+            ExecStart=/bin/sh -x -c \
+              'until curl --silent --fail https://status.github.com/api/status.json | grep -q \"good\"; do sleep 2; done'
+
     - name: docker.service
       drop-ins:
         - name: 50-weave-kubernetes.conf