As 284, part 1 - find a created server on Nova (ready for review)

This change is bigger than I thought, so I am trying to break it up into bits.  This part just provides the functionality to check Nova for an exact server that matches a given server config, created around a certain time.

Question for the next part - should we wait at all after getting a 500 or 401 response back from Nova from creating a server, before checking Nova to see if a server was created? 
But treq is a mock object, so `self.treq.json_content` should have returned a mock object, not the json blob.

Oh.  I it's a magicmock object, which has all the magic methods like `__getitem__` implemented, so indexing on it worked.  That's why the tests passed...