Plumb node labels through salt

Expose node-labels flag on kubelet for gce via a startup script
env var.

cluster/common.sh

@@ -600,6 +600,11 @@ KUBEPROXY_TEST_LOG_LEVEL: $(yaml-quote ${KUBEPROXY_TEST_LOG_LEVEL})
 EOF
     fi
   fi
+  if [ -n "${NODE_LABELS:-}" ]; then
+      cat >>$file <<EOF
+NODE_LABELS: $(yaml-quote ${NODE_LABELS})
+EOF
+    fi
   if [[ "${OS_DISTRIBUTION}" == "coreos" ]]; then
     # CoreOS-only env vars. TODO(yifan): Make them available on other distros.
     cat >>$file <<EOF