Rewrite ContentViewTestCase

* Drop an unnecessary `@ddt` decorator.
* Make the test code more straightforward.

Rather magically, the test now succeeds.

    $ nosetests tests/foreman/api/test_contentview.py:ContentViewTestCase
    .
    ----------------------------------------------------------------------
    Ran 1 test in 19.436s

    OK

This fixes a Jenkins failure.
```python
>>> system_id = entities.System().create()['id']
>>> system_attrs = entities.System(id=system_id).read_json()
>>> 'organization' in system_attrs
False
>>> 'organization_id' in system_attrs
False
```