Support online-maintenance in hroller

Make hroller take into account the nodes (redundant) instances
will be migrated to. This be behavior can be overridden by the
--offline-maintenance option which will make hroller plan under
the assumption that all instances will be shutdown before starting
with the rolling reboots.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Thomas Thrainer <thomasth@google.com>