WiP: Added apiserver endpoint for reboot

This is the second pull request for the reboot implementation. Please note that this is still a Work in Progress and a followup commit will be done for cleanup.

This branch depends on: https://github.com/juju/juju/pull/707
yeah, I don't think machines can reboot themselves. Or, well, hmm, they probably could/should -- if they themselves install something that requires a reboot -- so having this is fine, so long as it's commented as currently unused (with explanation of why it's there) -- but the main use case is for a unit requesting a reboot of its host machine. That's the one we should definitely have tests for.