Update the DD of OOB with gnt-cluster epo

This command came up in a local discussion about how much logic we
should put into gnt-node power off. We decided to go with a separate
convenience command which takes care of all the steps through the
procedures to turn down the cluster into a state suitable for a shutdown
of the underlying machine.

Signed-off-by: René Nussbaumer <rn@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>