Improve performance of the debian-etch-debootstrap OS

Improve performance of the debian-etch-debootstrap OS

This patch enables the debian-etch-debootstrap instance to generate (and
subsequently use) a cache file on the first install. Because, even with
a very fast mirror, debootstrap itself is slow, saving the installed
image and afterwards just untar-ing that over the filesystem is much
faster.

The patch also adds a 'version' file so that users can see what they
have installed (maybe in the future we could use require this file so we
know what versions of the OS we have installed).

Reviewed-by: ultrotter