[ament_pep257] make pep257 default in ament_pep257

It is very confusing that `ament_pep257` does by default NOT check for `pep257`, but instead for some seemingly arbitrary list of pydocstyle error codes, see https://github.com/ament/ament_lint/blob/rolling/ament_pep257/ament_pep257/main.py#L77

Why is some `--convention pep257` parameter necessary to actually really check for `pep257`? In my opinion it should not be called `pep257` if it doesn't check for `pep257` by default.

Does anyone know why this is not the case? I'd create a PR to change the default, but I'm not sure about implications on the ROS2 build farm.