builder/digitalocean: use DigitalOcean image ID for artifact Id()

The DigitalOcean image ID is easier to use than `region/slug` when using Packer as part of a build process, which will probably involve some invocation of the DigitalOcean API.

I'm not sure if there was a good reason for using `region/slug` (based on https://github.com/mitchellh/packer/commit/2cad46aa1fb16ffe1d7bd176050c578004bd10fb, it seems pretty arbitrary).