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
This is asserting that it was called once, as opposed to more than once (which would happen on retry), but thank you for pointing this out!   I should change this to a more meaningful assertion with checking the arguments and such.