Refactored existing API entities path tests.

Since we now have a few `orm.Entities` where we're overriding their
`path` methods, I figured we should refactor our existing `PathTestCase`
tests to make it easier to add test coverage for any future changes.

With the exception of `orm.entities.ForemanTask`, previous existing
tests have been refactored.
Refactor module tests.robottelo.test_entities

Make the following changes:

* Set class variable `PathTestCase.longMessage` to `True`, and rewrite several
  assertion error messages to take advantage of the change.
* Rewrite a significant number of docstrings, and strip out several spurious
  comments.
* Extend the set of assertions in `test_path_with_which`.
Merge pull request #1213 from omaciel/refactor-api-path

Refactored existing API entities path tests.