Allow the Crowbar core to turn idle nodes off.

If a node has been fully discovered (or had its operating system
installed), we know how to turn it back on, and it has nothing else to
do, turn the node off.  The framework will turn the node back on when a
node role bound to it is committed.
Merge pull request #174 from VictorLowther/turn-idle-nodes-off

+2 Validated on physical nodes.  POWER OFF AFTER DISC, RESTART AUTO WHEN ADDED TO DEPLOY.
Allow the Crowbar core to turn idle nodes off.