juju reboot support

api for unit agent request shutdown on self machine (triggered by hook exit value)
api for unit agent to watch for shutdown on machine
  -> on receipt
     stops further hook exec / blocking on the current one if any
     signals ready for shutdown

api for unit agent to signal ready for shutdown
api for machine agent watch for shutdown
  -> on shutdown recieved
      -> disables unit deployment
      -> informs all units of pending shutdown [ could also be done by requesting agent, but supervision tree looks nicer from child->parent-> children]
      -> initiates watch on all units on machine to reach shutdown ready state
      -> upon all units marked ready for shutdown, restarts the machine.
   

also needs to execute stop hooks