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? 
I'm assuming that @cyli is referring to entropy available to guests. If everyone gets all-zero bits, we're going to get an awful lot of collisions :)

Collisions are 2**(n/2). For 4 bytes == 32 bits truly random bits, 2**(16) == 65536. Or, to put that into perspective, if you make fewer than ten thousand machines (9300), collisions are already a non-improbable event (1%), and the curve doesn't get steep until way past that. Unless there's a good reason not to, I'd recommend a good deal more bits.

I tried to find information about how this works on Xen (I was hoping the answer was "virtio-rng", but can't really find anything confirming that.)