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
Needs copyright:
```
// Copyright 2014 Canonical Ltd.
// Licensed under the AGPLv3, see LICENCE file for details.
```