Update API host tests

Now to create a host medium_id is required. The tests was updated to
create a host with the extra required field.
Merge pull request #935 from elyezer/api-host-test

Update API host tests
Add class `Model` to module `robottelo.entities`

This new class represents a "Model" entity in the Satellite application.
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

Do not implement any methods. Instead, simply set up their basic structure and
write docstrings.
Implement method `attributes`

Method has only an extremely bare-bones implementation. It is only capable of
populating entities that contain nothing but string-based attributes.