Add option --one-step-only to hroller

Add a new option to hroller to only output information about the first
reboot group. Together with the option --node-tags this allows for the
following work flow. First tag all nodes; then repeatedly compute the
first node group, handle these nodes and remove the tags. In between
these steps, other operations can be carried out on the cluster.

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