Add preseed command to comment out cdrom in /etc/apt/sources.list.

[Line #44](https://github.com/socialally/bento/blob/ad4c9c59d6c3b666e07ecbaefad0da49a2bea3d7/definitions/debian-7.1.0/preseed.cfg#L44) comments out the cdrom source in /etc/apt/sources.list, resolving BENTO-82.

I created a new preseed file rather than modifying the shared debian/preseed.cfg file as this fix is only necessary for the definition for Debian 7, as far as I am aware.