As 284, part 2 - check for server on nova error

This part actually does the create server retries after checking to make sure no server was created.  This is based on https://github.com/rackerlabs/otter/pull/647
I think this errback warrants a real function, preferably one with a docstring. I'm guessing that you're relying on f.trap returning None, that being falsey, and "and" evaluating to the first truthy expression. it works, but please don't do that :)