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 :-)
I'm actually a fan of returning `response.json()` in entity methods such as `ContentViewVersion.promote`, `ContentView.publish` and `ForemanTask.read`. I think that returning JSON data instead of a response object is good because it promotes proper layering of code responsibilities. In other words, doing this makes for looser coupling.

Imagine, for a moment, that we decided to throw away `requests` and use a different library for making HTTP(S) requests. The entities themselves would need to be updated to use the new library, of course, as they are the ones making HTTP(S) calls. But what about the unit tests, smoke tests and so on? Should all of them need to be changed too? In an ideal world, test code only uses entities and entity methods, and never touches `requests` directly.

Of course, that's not the case right now. There's quite a bit of test code that uses `requests` directly. But this is the ideal I'm aiming for.