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.
Nitpick, non-blocking: since you're using super in the subclass, and future callers are technically allowed to muck with the MRO, consider using super all the way up. (although that only works per chance if the superclasses don't themselves use super correctly, but that's just Python brain damage :))