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.
It may be a good idea to update the docstring of this test to clarify what it actually does. It seems that there's three ones here that the implementation probably doesn't care about, but `"PENDING_UPDATE"` looks special. It seems this is only used when deciding whether or not to log when getting a 422, because the comination of 422, PENDING_UPDATE is not supposed to happen.