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.
I think that's doable. The body of `publish` could look like this:

```python
response = client.post(...)
response.raise_for_status()
if response.status_code != httplib.ACCEPTED:
   return None  # raise APIResponseError?

if synchronous is True:
    return ForemanTask(id=response.json()['id']).poll()
return response.json()
```