simplify digitalocean_droplet.image loading

If a droplet's image slug is empty and its image id is empty, then the
image attribute should be empty, so we may assign from either. So it is
unnecessary to check if the image id is empty.

* remove unnecessary check for emptiness of image id
* reverse order of the conditions for assigning the image attribute,
  with the default case (using the slug) first, and the fallback case
  (using the id) second
Merge pull request #617 from josephholsten/simplify-do-droplet-image-loading

simplify digitalocean_droplet.image loading