AS-288: Check deleted CLB when adding node (review)

This will reduce unwanted retries which will in-turn succeed invalid load balancer tests. I have factored out the checking of deleted CLB in separate function and used it in add and remove node code.
Yeah. I don't like 
```python
codes = iter([422, 422, 404])
self.treq.post.side_effect = lambda *_, **ka: succeed(mock.Mock(code=next(codes)))
```
code everywhere. It would be ideal to have it in `StubTreq`. So, either I wait for that to be merged and use that throughout or merge this with these changes and cleanup later on. I am leaning towards option 2 :).