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
This file is compiled only under windows. Also the path that is passed in will have the drive letter every time. So it is guaranteed to replace that one every time.