As 282, part 2 - check for server on nova error (pending merge pending QE ok)

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

Mergable once:
- [x] existing integration tests when creating servers pass
- [x] manual testing to make sure that if nova returns an error otter does not crash to the ground - make sure #682 still passes on this branch