Added preseed file for installation.

The preseed.cfg file contains options for automating the
ubuntu install process.

For more information see the:

* [Ubuntu 12.04 preseed documenation](https://help.ubuntu.com/12.04/installation-guide/i386/preseed-using.html), or
* [Official Debian instructions](https://wiki.debian.org/DebianInstaller/Preseed).