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
Same thing here as above, this time the function is only called under Windows, but this time it's format is file://foo/bar, so again it is guaranteed to not mess up the url since it will replace the dash in file-// after the first pass.
But sure, I will add some tests