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
and shouldn't we be handling a shutdown differently to a reboot here too? or are we looking up the expected action again inside finalize? That seems a bit off.