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:
>
> > +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()
> > +		if valid := jenvFilename.MatchString(filename); valid {
> > +			extension := filepath.Ext(filename)
> > +			name := filename[0 : len(filename)-len(extension)]
> > +			envs = append(envs, name)
> > +		}
> > +	}
> > +	sort.Strings(envs)
>
> After some time to think about it, I think I suggested incorrectly.
>
> We shouldn't push presentation code deeper than it needs to be. The 
> tests should be checking using jc.SameContents, and the code that 
> outputs it should handle the sorting.
>
> —
> Reply to this email directly or view it on GitHub 
> <https://github.com/juju/juju/pull/35/files#r13526678>.
>