Add OS to docker info

Docker-DCO-1.1-Signed-off-by: Tibor Vass <teabee89@gmail.com> (github: tiborvass)

This PR is to add the Operating System (that is, Linux distro) to the docker info output.
This is towards asking for a single command (`docker info`) when debugging issues.
Old thread: http://stackoverflow.com/questions/20010199/determining-if-a-process-runs-inside-lxc-docker
Newer thread: http://stackoverflow.com/questions/23513045/how-to-check-if-a-process-is-running-inside-docker-container

@crosbymichael Let me know if you disagree with this or if you think there's a better way.