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 doesn't do what you think it'll do.

If I had the name "/foo/bar/test-one", that would make the resulting fullpath "/foo/bar/test:one"

If the concern is keeping the first colon if it exists, then this should be found and recorded explicitly.  If it is about the path value of the fileStorageReader, then again, this should be done elsewhere.  This method is just a bit messed up.