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.
@tianon I updated the tests with your weird Gentoo example to be a little future proof.