Add windows userdata

This is a work-in-progress branch.

This branch adds userdata support for windows systems.
> Can't DataDir and LogDir be calculated later, in the renderer, based on the series?

I think we can avoid adding the series parameter to NewMachineConfig, and set them to "" here. Then later, in ConfigureBasic/ConfigureJuju, we can inspect the Tools field to get the series and work out the right defaults from there.