Add node_labels variable in workers modules to set initial node labels (#550)

* Also add `worker_node_labels` variable in `kubernetes` modules to set
initial node labels for the default workers

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

@@ -61,6 +61,9 @@ systemd:
           --lock-file=/var/run/lock/kubelet.lock \
           --network-plugin=cni \
           --node-labels=node.kubernetes.io/node \
+          %{ for label in split(",", node_labels) }
+          --node-labels=${label} \
+          %{ endfor ~}
           --pod-manifest-path=/etc/kubernetes/manifests \
           --read-only-port=0 \
           --volume-plugin-dir=/var/lib/kubelet/volumeplugins