environs: fixes for Windows

This PR makes some transparent changes to implement filestorage on windows.

We cannot write files with : on the disk so we write everything with a -. This also means that anything that's read from the disk also has to contain dashes.

Depends on https://github.com/juju/utils/pull/45
ok, I have a couple of comments here:
1) hardcoding a unit does not seem a very good idea and I believe it is error prone in the future.
2) I have seen these replaces too many times this might be good as an idempotent function of some sort or convert this string into a more complex type that can translate itself to something that suits the OS underneath.