Build nginx_upload_module as a dynamic module

It's now being maintained again and support was added in fdintino/nginx-upload-module#98. This would allow us to stop building full replacement nginx packages, at least for nginx 1.9+.

Note that the upload module is not used by the UI's chunked uploader. It would be used by scripts if directed to upload to the upload module target (`/_upload` in the docs) although I don't think this has been explained in the past. For most deployments, you probably do not need the upload module, and therefore, a "special" nginx anymore. It is also possible to configure Pulsar stage-out to use the upload module (which is the reason I still need it).