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
So, to be clear, the idea at this point is that if trying to create a server failed with something other than a 400 response, we're now checking if that actually created a server or not; if it secretly did, we're going to pretend the error never happened, otherwise, we raise the original failure?

If so: that's really clever and I love it, but I think it could afford to be documented in the intermediate docstrings.