packer: support different Debian + grml-debootstrap releases

* update Grml ISO to version 2014.11
* instead of running 'make packer' now it's possible to execute
  "make $debian_version" with support for lenny, squeeze, wheezy,
  jessie and stretch as $debian_version (lenny lacking support for
  Virtualbox Guest Additions though)
* setting GRML_DEBOOTSTRAP_VERSION allows to control the
  grml-debootstrap version that should be used, defaults to the
  latest CI build from jenkins.grml.org