Change docker_gwbridge subnet

Change-Id: Ibcedd2dce7a117e2cf96c4379c156d54aaec0299
diff --git a/docker/swarm/master.yml b/docker/swarm/master.yml
index bb2d83b..fba9af3 100644
--- a/docker/swarm/master.yml
+++ b/docker/swarm/master.yml
@@ -1,4 +1,8 @@
+classes:
+  - system.docker.swarm.network.gwbridge
 parameters:
+  _param:
+    docker_gwbridge_subnet: 10.20.0.0/16
   docker:
     swarm:
       role: master