Add configuration file into /etc/default/ for ubuntu and debian packages

This PR is based on a small change that I have found useful in my local environment.  Like what is used in many other ubuntu/debian packages, I have created a /etc/default/docker configuration file that is sourced by a modified version of the upstart script.  This allows me to properly setup the environment for the docker daemon to run.  Specifically, I use this to setup custom proxy variables for docker to use.

While I am not currently using the debian package, I have made similar changes to it as well.

If you find this PR contains something docker would benefit from, please feel free to take the patch as-is, or make any necessary changes.  All of my changes are noted in the commit message.