Allow for artifacts that don't specify a path for uploading to S3
Allow for artifacts that don't specify a path for uploading to S3

By not passing a path to the done() callback,, an artifact is declaring
that there's no output to upload to S3. This is handy for artifacts
that have their own storage (like docker images).

As a result it's also not necessary to specify a region/bucket.

aliak00/kosmo,aliak00/kosmo,comoyo/nova,comoyo/nova