allow legacy disk shares and correctly store thumb

legacy objects may be on share other than the one configured in
application.yml if they are don't try to force anything to the
 currently defined share.  also although the datatream is
thumbnail.png the directory of the thumbs is thumb_png
I'd suggest:

```ruby
      if datastreams[name] && datastreams[name].dsLocation
        datastreams[name].dsLocation
      else
        File.join(remote_root, file_path(name, extension))
      end
```