Add digital-ocean cloud support.

Same as [previous PR](/ansible/ansible/pull/3188).

Add support for [Digital Ocean](https://www.digitalocean.com) cloud provider; this module rely on [dopy](https://pypi.python.org/pypi/dopy).

It allows:
- creation / deletion of droplets (servers) based on:
  - size
  - region
  - image
- add / remove SSH keys (those keys get then added to the root user of the newly created servers)
- optional wait until server is created