Updated docker upstart script (should fix service docker restart).

This PR updates the template for the upstart service in `/etc/init/docker.conf to match the upstream version.

The story: I was wondering why my docker containers poofed each time I ran `service docker restart`. Before this PR, typing `sudo service docker restart` would immediately restart the daemon but the containers are gone in `docker ps`. The reason is that upstart did not detect the end of docker daemon (tracking the pid of the shell wrapper) so it would not emit the "started" event and the container services would not be stopped cleanly nor restarted cleanly.

After this PR, running `sudo service docker restart` with containers are running should take a little longer, while docker shuts down the running containers gracefully.