Add a topology list accessor

[sajith]

We do this in sdx-controller:

        for num, val in enumerate(temanager.topology_manager.topology_list):
            logger.info(f"TE topology #{num}: {val}")

https://github.com/atlanticwave-sdx/sdx-controller/blob/b65b5b228c20e962eb1c4666a4f6adae480326d3/swagger_server/handlers/connection_handler.py#L123-L124

We probably should be using a getter method there.