providers/digitalocean: fix acceptance test suite for running
providers/digitalocean: compat for changes to API for droplet size

Upstream in the digitalocean API, the size slug became a size
string, breaking pearkes/digitalocean.

https://github.com/pearkes/digitalocean/pull/1

This fixes compatibility for changes to that library.

closes #510
Merge remote-tracking branch 'origin/master' into issue-518