Rename module robottelo.models to robottelo.orm

This renaming has been done for the sake of clarity. Module robottelo.orm
contains components for mapping Python classes to Satellite entities, and module
robottelo.entities contains complete mappings of Python classes to Satellite
entities.

Also:

* Fix imports in module robottelo.entities to accomodate the rename
* Flesh out docstrings in module robottelo.entities

Related to #916