WiP: Add machine reboot support

This PR adds changes in the machine agent needed to support reboot.

Depends on:

https://github.com/juju/juju/pull/707
https://github.com/juju/juju/pull/728
https://github.com/juju/juju/pull/744
https://github.com/juju/juju/pull/782
can we pull this magic string out into a constant in the `container` package or something?