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
This will be rebootuniter.UniterRebootAPI when used by other pacakges. Consider rebootuniter.API. Either way, comment should start with func name.