Updating to support v1 of compute engine api

Hi Doug,
As the v1beta15 of the compute engine API will be deprecated in January, I've updated the code to support v1.
The main change is that it no longer supports scratch disks so a persistent disk needs to be created form an image.  
I've hard-coded a default source image (current latest Debian) but it can be overridden via the extensibility pattern.
I've also set it up to delete the disk when the VM is stopped to avoid lots of old disks kicking around given we are essentially using them as scratch disks. 
I hope that all makes sense, let me know if you have any questions or issues. 

Ben