Add sample cloud-config for testing MCM independently.

[hardikdr]

What would you like to be added: In order to test MCM independent of Gardener, the created machines has to join the cluster. And this requires the machine to be backed by an appropriate cloud-config. We should add a sample/bare-minimum cloud-config with place-holders for any of the known-OSes in the documentation, to make the testing easier for the community.

[hardikdr]

@zuzzas Did you prepare any sample cloud-config during your tests back then, which could be used here in documentation? :)

[prashanth26]

/priority 4 /effort 1d

[himanshu-kun]

/close in favor of #770