Sort reboot groups by size

Make hroller output the node groups not containing the master node
sorted by size, largest group first. The master node still remains
the last node of the last reboot group. In this way, most progress
is made when switching back to normal cluster operations after the
first reboot group.

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