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.
Because this has change that removes logging of all responses. It now only loggs unexpected errors, i.e. anything other than 422 PENDING_UPDATE. That logging is tested in `test_removelb_retries_logs_unexpected_errors`