reboot worker

This is the final PR for the reboot functionallity. It add a Reboot worker that waits for the machines reboot flag to be changed, and reacts.

This branch depends on:

https://github.com/juju/juju/pull/707
https://github.com/juju/juju/pull/728
https://github.com/juju/juju/pull/744
This fix seems to imply a test is missing.