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 :-)
Yes, we can do that. However, it's worth noting that there are already tests in place which do so. See [here](https://github.com/omaciel/robottelo/blob/master/tests/foreman/api/test_system_v2.py#L161-L171). Thus, I think making that check here is counter-productive: it can cause the test to fail before getting to the point of the test (can a subscription be made?), and making such a check doesn't improve our test coverage at all.