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? 
Very minor nitpick: I feel this statement should be just above `d.addCallback(get_server)` to localize the fact the content got is passed to `get_server`. Just a suggestion. You don't have to change it :)