Minor changes to the box configuration

* Removed Chef
* The Debian ISO mirror can be overriden by an environment
  variable (DEBIAN_MIRROR).
* The Debian version can be overriden by an environment
  variable (DEBIAN_VERSION).
* The disk size defaults to 10 Gbytes but can be overridden
  by an environment variable (DISK_SIZE).
* The path to the bsdtar program can be overridden by an
  environment variable (BSDTAR)
* The installed sources.list now uses the US Debian mirror.
* Added python-dev, python-setuptools, and tmux to the set
  of default packages.
* Removed puppet and screen from the set of default packages.