Fallback to add-on name if attachment_name is None

When trying to remove the SSL add-on from one of my apps, the library used `None` as add-on name. That's because rather than sticking to `self.name`, the code replaces it with `self.attachment_name`, which is `None` in this particular case. My change defaults to `self.name` in case `self.attachment_name` is `None`