Add GCE auth support for GKE-based Kubernetes

Change-Id: I757b6fd0111945373615f4c509db21bba33f3a02
diff --git a/metadata/service/client.yml b/metadata/service/client.yml
index 1822dd8..ccd42f7 100644
--- a/metadata/service/client.yml
+++ b/metadata/service/client.yml
@@ -21,5 +21,6 @@
         config:
           cluster: {}
           user: {}
+          gce_service_token:
       repos: {}
       releases: {}