Automatically append _id and _id to field names

Entities have OneToOneField and OneToManyField names in this format:

    organization = orm.OneToOneField('Organization', required=True)

However, the Foreman API expects the '_id' and '_ids' suffix to be appended to
these same fields:

    {'organization_id': 15}

Insert logic into class EntityFactoryMixin that transforms field names as
appropriate.

Remove method Factory._get_field_names. This method added complexity to the
behaviour of Factory, and that complexity is unneeded, as no client code
actually relied on that logic in any good way. To accomodate this change, remove
the 'fmt' argument from Factory.attributes, Factory.build and Factory.create.

Rename two more methods:

* Factory._get_path to Factory._factory_path
* Factory._get_values to Factory._factory_data

The Factory class is used as a mixin (via EntityFactoryMixin) in numerous,
numerous Entity classes. Renaming these two fields helps avoid any potential
name conflicts with child classes, and also reduces confusion when reading
through EntityFactoryMixin.