Fix the gateway and router services for CDAP 2.6+

Starting with CDAP 2.6, there is no longer a `cdap-gateway` Thrift service. There is still a `cdap-gateway` package, but it installs only the `cdap-router` service. On older versions, the `cdap-gateway` package installed both the `cdap-router` and `cdap-gateway` services.
Not sure what this is.. but looks OK.