Replace StorageNamespaceList.to_str with __repr__.
Replace StorageNamespaceList.to_str with __repr__.

This fixes the config value output when running SocorroApps to have a readable
value for options using this class.

lonnen/socorro,mozilla/socorro,mozilla/socorro,lonnen/socorro,lonnen/socorro,mozilla/socorro,mozilla/socorro,lonnen/socorro,mozilla/socorro,mozilla/socorro