Finish replacing core data objects with 'attrs' based classes

[alikins]

Most of the core data objects used in mazer are now based on 'attr' based classes, but some still need to be replaced with 'attr' based classes.

This includes:

• [x] ansible_galaxy.flat_rest_api.content.Content
• [x] ansible_galaxy.flat_rest_api.content.InstalledContent
• [ ] ansible_galaxy.config.config.Config

But mainly ansible_galaxy.flat_rest_api.content.*

[alikins]

Fixed by https://github.com/ansible/mazer/pull/156 Also kind of a dupe of https://github.com/ansible/mazer/issues/127