Adding fix for shutdown problems

See
https://github.com/webcoyote/packer-templates/blob/master/centos-6.4-x86
_64-netboot/scripts/zerodisk.sh#L5-L14
Adding fix for shutdown

See
https://github.com/webcoyote/packer-templates/blob/master/centos-6.4-x86
_64-netboot/scripts/zerodisk.sh#L5-L14