Make API docs include more modules

Make the API docs cover the following modules:

* `tests.foreman.api.test_host_v2`
* `tests.foreman.api.test_model_v2`

Incidentally, this fixes a broken link in the API docs for module
`robottelo.factories`.
Merge pull request #945 from Ichimonji10/docs

Make API docs include more modules
Add more entities and factories

Add the following classes in module `robottelo.entities`:

* `Organization`
* `Product`

Add the following classes in module `robottelo.factories`:

* `OrganizationFactory`
* `ProductFactory`

The "Product" entity includes a required foreign key field, so factories must be
capable of generating values for those fields. Update function
`robottelo.factories._get_default_value` with the ability to handle fields of
type `orm.fields.OneToOneField`. Write unit tests to check this functionality.