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.
Non-blocking - might be useful to actually include the error code and message for log purposes, in case support is looking this, and/or distinguish between the node being deleted vs the load balancer not existing/being deleted.