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
Ahh... *this* is where we prevent nested lxc. Heh.

Regardless -- shouldn't there be a similar check for KVM support? This change feels like it deserves its own separate CL (because apart from anything else it needs its own tests).