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.
+1.

As a present here's Fedora 20

```
NAME=Fedora
VERSION="20 (Heisenbug)"
ID=fedora
VERSION_ID=20
PRETTY_NAME="Fedora 20 (Heisenbug)"
ANSI_COLOR="0;34"
CPE_NAME="cpe:/o:fedoraproject:fedora:20"
HOME_URL="https://fedoraproject.org/"
BUG_REPORT_URL="https://bugzilla.redhat.com/"
REDHAT_BUGZILLA_PRODUCT="Fedora"
REDHAT_BUGZILLA_PRODUCT_VERSION=20
REDHAT_SUPPORT_PRODUCT="Fedora"
REDHAT_SUPPORT_PRODUCT_VERSION=20
```