Whenever bootstrap files hosted in the http_directory are accessed, it should show in the logs

Here is a simple suggestion to save us lots of time in debugging.

In images like `virtualbox-iso`, one can host files using the `http_directory` parameter.

These files are are, for example, "install settings" used in debian and ubuntu.

On a good day, everything works as expected.

On other days, it would be great if the internal HTTP server on packer showed some very basic access logs, so we could be sure our ISO image is hitting the HTTP server to download the files and this way narrow down whatever error we are having to "before" or "after" downloading the HTTP file.