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? 
@manishtomar 
> I wonder how much otter can work if Nova returns wrong data

Reach had previously encountered Nova bugs, such as Nova occasionally returning a zero servers when listing all servers for a tenant.  Also when testing this I also realized that Nova returns a 500 if the regex is malformed, rather than a 400.  It's possible there might be an occasional Nova bug - obviously if they return zero servers, we can do nothing, but if they return an incorrect list of servers if we don't check and log we'd have no way of knowing.