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](http://tickets.opscode.com/browse/BENTO-82?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel).

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.