Add design doc for virtual(ised) clusters

I am currently able to run a 2-node virtual cluster on my machine,
with a very ad-hoc setup. But the results show clearly that this is
doable, and that given the right tools, setting up such a cluster will
be quite easy.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>