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
ErrPerm is just a replacement for NotFound when the client doesn't have any reason to know about the existence or status of the tag under consideration. It's not just a catch-all -- it's about detecting requests that *cannot* be correct and leaking the absolute minimum of information.