Show CoreOS version in motd

Add CoreOS version to ```motd``` by parsing the ```VERSION``` line from ```/etc/os-release```.

Reasoning: I find myself quite often in the situation that I would like to quickly check which version a node is running. This requires me to ssh into a node and ```cat /etc/*release```. This change makes it more convenient by showing the version in the motd.

How about just add `source /usr/lib/os-release` (the real location of that file, /etc just has a symlink) and use `${VERSION}` down below.