Add "virtio" and "private_networking" options for DigitalOcean droplets

The dopy manager defaults to False for virtio, but the DigitalOcean web interface defaults to True, which is a pretty safe default and sensible considering the performance gains.

Private networks are a new feature currently available only in the NYC2 region as of Oct 2, 2013.

NOTE: dopy >= 0.2.2 is required for these options to work.