Add class `Model` to module `robottelo.entities`

This new class represents a "Model" entity in the Satellite application. For
reference material, see `apidoc/v2/models` on a Satellite deployment.
Add new module `robottelo.factories`

Do not include any useful code in this module. Instead, simply write out a
docstring.

Make the API docs include this new module.
Add minimal content to module robottelo.factories

Create the following classes:

* Factory
* HostFactory
* ModelFactory

Implement methods `__init__` and `attributes` on class `Factory`.