Extend the node description by isMaster

Extend the description of the node by the property
of being the master node; also provide and appropriate
setter function. This property will be used, e.g., by
hroller to schedule reboot of the master last.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>