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.
Personally, I'd prefer to see consistency in syntax among the various cloud providers. It looks like most of the cloud providers expect attributes such as `:public_ips` to be provided in the hints file. I see your comment to prefer actual IP address over hints. What about using the hint data as fallback?