Changes the replicas of an RC based on a manifest.
        Note that it defaults to 0, meaning to effectively disable this RC.

        :Parameters:
           - `manifest_filename`: The manifest file containing the ReplicationController definition, for example: `manifest/nginx-webserver-rc.yaml`
           - `namespace`: In which namespace the RC is, defaulting to `default`
           - `num_replicas`: How many copies of the pods that match the selector are supposed to run

def scale_rc(self, manifest_filename, namespace="default", num_replicas=0):
        """
        Changes the replicas of an RC based on a manifest.
        Note that it defaults to 0, meaning to effectively disable this RC.

        :Parameters:
           - `manifest_filename`: The manifest file containing the ReplicationController definition, for example: `manifest/nginx-webserver-rc.yaml`
           - `namespace`: In which namespace the RC is, defaulting to `default`
           - `num_replicas`: How many copies of the pods that match the selector are supposed to run
        """
        rc_manifest, rc_manifest_json = util.load_yaml(filename=manifest_filename)
        logging.debug("%s" %(rc_manifest_json))
        rc_path = "".join(["/api/v1/namespaces/", namespace, "/replicationcontrollers/", rc_manifest["metadata"]["name"]])
        rc_manifest["spec"]["replicas"] = num_replicas
        res = self.execute_operation(method="PUT", ops_path=rc_path, payload=util.serialize_tojson(rc_manifest))
        try:
            rc_url = res.json()["metadata"]["selfLink"]
        except KeyError:
            raise ResourceCRUDException("".join(["Sorry, can not scale the RC: ", rc_manifest["metadata"]["name"]]))
        logging.info("I scaled the RC %s at %s to %d replicas" %(rc_manifest["metadata"]["name"], rc_url, num_replicas))
        return (res, rc_url)