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.
As another potentially fun example, here's mine from Gentoo:
```
NAME=Gentoo
ID=gentoo
PRETTY_NAME="Gentoo/Linux"
ANSI_COLOR="1;32"
HOME_URL="http://www.gentoo.org/"
SUPPORT_URL="http://www.gentoo.org/main/en/support.xml"
BUG_REPORT_URL="https://bugs.gentoo.org/"
```