config.vm.box_url can be a list/tuple of URLs, which is useful to specific
a locally cached copy. This is needed on slow connections, so that if it
fails, the download of jessie32.box does not have to start from the
beginning of the file again.
This also makes the file layout in git basically the same as the installed
file layout, using an examples/ dir. I'm not sure if config.buildserver.py
is an example conf file, or a conf file that is actually in use, so I did
not move it.
