Add a Reboot resource into core. (RFC code.)

For ease of testing, the actual reboot is commented out.
Agreed. We should definitely support why-run. 

Using the current why-run structure, the error will be displayed in the order reboot resource shows up in the resource collection. From a user point of view, it would be nice if we have an additional message at the end of the run saying `this node would reboot` in why-run mode.