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.
Yes, `create` should assign a value to this object's `id` attribute. Unfortunately, that's not going to happen for a little while yet.

First, #1631 should be merged. When that's done, this code can be rewritten slightly to look like this:

```python
org = entities.Organization()
org.id = org.create_json()['id']
```

Then, `EntityCreateMixin.create` should be fleshed out and made to act like [`EntityReadMixin.read`](https://github.com/Ichimonji10/robottelo/blob/master/robottelo/orm.py#L583-L603). After creating a new entity, `create` should construct a new object and populate its attributes using the contents of the server's response. That will take [quite a bit of work](https://github.com/SatelliteQE/robottelo/blob/master/tests/foreman/api/test_multiple_paths.py#L654) to do, but yes, it can happen.