Add element to modify /etc/apt/sources.list in dib

I use Ubuntu 13.10 64B as dev OS. the default /etc/apt/sources.list is slow in my network environment. In my dev OS, I change it to local mirrors, e.g. http://mirrors.sohu.com/ubuntu/.
For the target cloudimg, there is no element in dib to change the default sources.list, it will be a long time for "apt-get update" in target .
Although the http_proxy can accelerate the build process by hitting the squid cache, but for the first time, all the things will be downloaded from source. It's better to add an element to modify the /etc/apt/sources.list for cloudimg.