Istio integration (#421)

This the very first step for fission to integrate with Istio, which is an open platform to connect, manage, and secure microservices. With Istio, users are able to monitor functions usage and trace requests latency through dashboards. For more information, please visit http://fission.io/docs/

executor/fscache/functionServiceCache.go

@@ -163,7 +163,9 @@ func (fsc *FunctionServiceCache) Add(fsvc FuncSvc) (*FuncSvc, error) {
 				err = nil
 			}
 		}
-		log.Printf("error caching fsvc: %v", err)
+		if err != nil {
+			log.Printf("error caching fsvc: %v", err)
+		}
 		return nil, err
 	}
 	return nil, nil