AS-288: Check deleted CLB when adding node - no tests (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. Tests are not changed yet. Kindly review it and I'll change tests after that.
I think this is still required.  I think the fix just makes it fail faster and with an obvious error, but it will still fail.