v2.0 API support

Digital Ocean changed their API to v2.0
https://www.digitalocean.com/company/blog/api-v2-enters-public-beta/

This adds support for v2 and replaces the old v1 API.
(Old API keys are now hidden and not readily accessible anymore)

Notable changes
--
To conform with the API documentation,
* provider.region uses slugs, such as `nyc2` instead of `New York 2`
* provider.size is now lowercase - `512mb`