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.
@manishtomar Hmm... I was originally  thinking CLBDeleted/CLBNodeDeleted also, but another possibility is that the load balancer simply never existed in the first place (404), so probably just including the RequestError would be more accurate.  :)