Update tests, docstrings, var and function names

* Update tests in module `tests.robottelo.test_factories` and
  `tests.robottelo.test_orm` with better assertions.
* Update docstring for class `robottelo.factory.EntityFactoryMixin`.
* Rename method `_is_required` to `field_is_required` in module
* `robottelo.factory.`
* Rename the variable `type_` to `field` in several locations.