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
I would prefer we have a cleaner way to simply get supported (and currently bootstrapped) containers, and just avoid running code, then to spam the logs with errors. There is some logic in the container brokers that tells you if KVM is supported. I would prefer we extend that with something like:

IsInstalled() (bool, error)

Then we can call that for each known broker and only try and list containers for that particular broker.