update packer to 0.5.1. fix template to fit new version

see https://github.com/mitchellh/packer/blob/master/CHANGELOG.md#051-01022014 for changelog'
download current packer version 0.5.1: http://www.packer.io/downloads.html
0.5.2 is near but with no breaking changes until now
make iso_checksum and debian iso version configurable over user variables
exclude vim and curl to own scripts.