WiP: Add machine reboot support

This PR adds changes in the machine agent needed to support reboot.

Depends on:

https://github.com/juju/juju/pull/707
https://github.com/juju/juju/pull/728
https://github.com/juju/juju/pull/744
https://github.com/juju/juju/pull/782
This is too much like magic IMO. If we're going to do this we should make sure we're using a definition that is guaranteed to match the actual service names -- this'll mean exporting something from the deployer code, I think.