Adds an external network gateway to the specified router

def add_gateway_router(self, router, network):
        '''
        Adds an external network gateway to the specified router
        '''
        router_id = self._find_router_id(router)
        net_id = self._find_network_id(network)
        return self.network_conn.add_gateway_router(
            router=router_id, body={'network_id': net_id})