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.
Ideally, it should be separate exceptions. Depending on the error responses there can be `CLBNotFound` (404), `NodeNotFound` (404), `CLBDeleting` (422 - PENDING_DELETE). @cyli and I had discussed about it and decided to just go ahead with above primarily because I was lazy to implement all the above :). This is the reason why the exception has `RequestError` in it.