test_cv_subscribe_system

Lets have another round and hopefuly prevent another bazzillion comments :-) :

1) if you thing that test_cv_subscribe_system attempts to test too much at once and I should first port the old promote/publish api tests before this one, tell me as well.

But I'd like to have this thing merged and then to move on to the rest of content-view stuff :-)
All usages of this method must be updated, as `response.json()` is now being returned instead of `response`. For example, `test_api_smoke.py` contains a usage of this method.