envions/configstore: Storage.List()

Add the List() method to the Storage interface. List lists all
the existing environments in storage.
Done.

> In environs/configstore/disk.go:
>
> > @@ -95,6 +95,23 @@ func (d *diskStore) CreateInfo(envName string) (EnvironInfo, error) {
> >  	}, nil
> >  }
> >
> > +// List implements Storage.List
> > +func (d *diskStore) List() ([]string, error) {
> > +	path := filepath.Join(d.dir, "environments")
> > +	var envs []string
> > +	files, err := ioutil.ReadDir(path)
> > +	if err != nil {
> > +		return nil, err
> > +	}
> > +	for _, f := range files {
> > +		filename := f.Name()
> > +		extension := filepath.Ext(filename)
>
> Use filepath.Glob(... "*.jenv") would be even better
>
> —
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> <https://github.com/juju/juju/pull/35/files#r13476555>.
>