digital-ocean: Add kubelet hostname-override flag

* Kubelets should register nodes via their private IPv4 address,
as provided by the metadata service from Digital Ocean
* By default, Kubelet exec's hostname to determine the name it should
use when registering with the apiserver. On Digital Ocean, the hostname
is not routeable by other instances. Digital Ocean does not run an
internal DNS service.
* Fixes issue where the apiserver can't reach the worker nodes. This
prevented kubectl logs and exec commands from working

digital-ocean/container-linux/kubernetes/cl/controller.yaml.tmpl

@@ -23,8 +23,11 @@ systemd:
       contents: |
         [Unit]
         Description=Kubelet via Hyperkube ACI
+        Requires=coreos-metadata.service
+        After=coreos-metadata.service
         [Service]
         EnvironmentFile=/etc/kubernetes/kubelet.env
+        EnvironmentFile=/run/metadata/coreos
         Environment="RKT_RUN_ARGS=--uuid-file-save=/var/run/kubelet-pod.uuid \
           --volume=resolv,kind=host,source=/etc/resolv.conf \
           --mount volume=resolv,target=/etc/resolv.conf \
@@ -48,6 +51,7 @@ systemd:
           --network-plugin=cni \
           --lock-file=/var/run/lock/kubelet.lock \
           --exit-on-lock-contention \
+          --hostname-override=$${COREOS_DIGITALOCEAN_IPV4_PRIVATE_0} \
           --pod-manifest-path=/etc/kubernetes/manifests \
           --allow-privileged \
           --node-labels=node-role.kubernetes.io/master \

digital-ocean/container-linux/kubernetes/cl/worker.yaml.tmpl

@@ -23,8 +23,11 @@ systemd:
       contents: |
         [Unit]
         Description=Kubelet via Hyperkube ACI
+        Requires=coreos-metadata.service
+        After=coreos-metadata.service
         [Service]
         EnvironmentFile=/etc/kubernetes/kubelet.env
+        EnvironmentFile=/run/metadata/coreos
         Environment="RKT_RUN_ARGS=--uuid-file-save=/var/run/kubelet-pod.uuid \
           --volume=resolv,kind=host,source=/etc/resolv.conf \
           --mount volume=resolv,target=/etc/resolv.conf \
@@ -48,6 +51,7 @@ systemd:
           --network-plugin=cni \
           --lock-file=/var/run/lock/kubelet.lock \
           --exit-on-lock-contention \
+          --hostname-override=$${COREOS_DIGITALOCEAN_IPV4_PRIVATE_0} \
           --pod-manifest-path=/etc/kubernetes/manifests \
           --allow-privileged \
           --node-labels=node-role.kubernetes.io/node \