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 Really all I'm doing is checking for `PRETTY_NAME`. What we could do however, is fallback to look for ID and VERSION_ID (or VERSION if no VERSION_ID), and concatenate them and use that instead. WDYT?