Adds private networking for Digital Ocean

Enables private networking for new droplets if the region supports it.
Fails gracefully if not: https://developers.digitalocean.com/droplets/.

> private_networking Optional, Boolean, enables a private network interface if the region supports private networking
Updates dop version to support private networking and scrubbing
Merge pull request #594 from Springest/private_networking_support

Adds private networking for Digital Ocean