Intermediate Lifecycle Integration

i.e. make it possible for the poweroff button to cause the system to shut down via the NSM, validating that the NSM is shutting down the legacy apps.

There are two approaches, here:
1. Have the PowerOff.service systemd unit send a D-Bus message to the NSM to set the node state to shut down. This is slightly perverse, as only the NSC and the NSM's state machine should be setting the node state.
2. Implement a new state machine which is capable of detecting that the PowerOff button has
been pressed, via some form of IPC.