add missing service definition/url for gwc

required to have the gateway start, cf georchestra/ansible#128 (comment)

gateway/routes.yaml

@@ -71,6 +71,7 @@ georchestra.gateway.services:
   datahub.target: http://localhost:8280/datahub/
   geonetwork.target: http://localhost:8280/geonetwork/
   geoserver.target: http://localhost:8380/geoserver/
+  geowebcache.target: http://localhost:8380/geowebcache/
   header.target: http://localhost:8280/header/
   import.target: http://localhost:8280/import/
   mapstore.target: http://localhost:8280/mapstore/