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
I will add a comment. This will be used when we will implement container support for windows. installing the hyper-v role on a windows server will require a reboot.