Change expected HTTP status codes to 201

The HTTP 201 status code means that a resource was created. The tests which
attempt to create a resource should expect an HTTP 201, not an HTTP 200.
Merge pull request #938 from Ichimonji10/fix-test

Change expected HTTP status codes to 201
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 (inherits from Factory)
* ModelFactory (inherits from Factory)

Implement the following methods on `Factory`:

* `__init__`
* `attributes`

The new factories can be used for quickly building new entities on a Foreman
server.
Add test for method _get_protected_fields

Tested method is in module `robottelo.factories`.