Digital Ocean ohai/cloud support

This PR does a few things:

* Implements a Digital Ocean ohai plugin
* Adds support for said plugin to the cloud plugin
* Adds support for said plugin to the cloud_v2 plugin

I will be issuing a separate PR to `knife-digital_ocean` for hint support.
In my experience with DO (and as far as I can tell via their docs), public IPs are always on `eth0` and private IPs are always on `eth1`. [Relevant docs](https://www.digitalocean.com/community/tutorials/how-to-enable-digitalocean-private-networking-on-existing-droplets).

The current functionality will use `eth0` if present/only a single interface is set (private IPs are simply set to empty arrays).

I didn't want to use hint information for networking since any IP changes may not be immediately reflected in the hint. I can do a search but then we'll need some way of specifying the IP class that corresponds to public vs private.