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.
I was going to suggest that since these correspond to the codes, maybe they can be defined as tuples of `(code, message)` and then `self.treq.post.side_effect` can be a function that sets `treq.content.side_effect` and also returns a code, but this may be too much effort considering #654 is almost in.