Expose OS name and OS version of the machines to prometheus.

[hardikdr]

What would you like to be added: We have been requested to expose the OS-name and OS-version of each machine to the Prometheus metrics.

Metric example:

machine{id="AAA", os="garden linux", version="1.2.3", ...} 1
machine{id="BBB", os="garden linux", version="1.0", ...} 1
machine{id="CCC", os="garden linux", version="1.2.6", ...} 1

We discussed, we could add a Version field in the MachineClass, which can then be used by the machine-controller to expose the metric.
cc @istvanballok @wyb1

[prashanth26]

/close in favour of #211 (comment)