Add /etc/default/docker to ubuntu package

* added packaging/ubuntu/docker.default
* modified packaging/ubuntu/Makefile to install packaging/ubuntu/docker.default into package as /etc/default/docker
* modified packaging/ubuntu/docker.upstart to source /etc/default/docker, if it exists
Add /etc/default/lxc-docker to debian package

* added packaging/debian/lxc-docker.default
* modified packaging/debian/lxc-docker.init to source /etc/default/lxc-docker, if it exists