digital ocean: added support for new private networking flag

I've added support for Digital Ocean's new private networking flag during node creation. This code checks against the region id to make sure it matches NY2, since this feature is only available there. Available region is encapsulated in a function so that new ones can be added as DO expands. 


The flag can be set in a profile like such:
    
    ubuntu_ocean_tiny:
        provider: dig-ocean
        image: Ubuntu 12.04 x64
        size: 512MB
        private_networking: True
        

When the node is deployed, you should see a new field in the api response called `private_ip_address`.