Adds an internal network interface to the specified router

def add_interface_router(self, router, subnet):
        '''
        Adds an internal network interface to the specified router
        '''
        router_id = self._find_router_id(router)
        subnet_id = self._find_subnet_id(subnet)
        return self.network_conn.add_interface_router(
            router=router_id, body={'subnet_id': subnet_id})