Correct RebootProgram usage

Correct RebootProgram logic when executed outside of a maintenance
  reservation. Previous logic would mark the node up upon response
  to the reboot RPC (from slurmctld to slurmc) and when the node
  actually rebooted, flag that as an unexpected reboot. This new
  logic checks the node's up time to not mark the compute node as
  being usable until the reboot actually takes place.
but 1866