Extract the systemd docs from various places and add a little more

@lsm5 @philips @jfrazelle I don't grok why the fedora `docker.services` file is different from the one in https://github.com/docker/docker/blob/master/contrib/init/systemd

if you're ok with it, I'll update the one in our repo - or maybe we should delete them? 
@lsm5 I am saying making a modification to the docker.service file in tree to have a final argument of ${DOCKER_OPTS}. Then the user can export an environment variable with this set. This is an alternative to /etc/sysconfig which uses systemd built-in stuff.