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? 
Another nitpick: The function can be named `got_server`. Even better, IMHO the function can be named based on what it is doing. In this it is checking if server exists. So, maybe `is_server_exists` or `match_server`?