natbox: fix NameError.

```
/opt/axsh/wakame-vdc/dcmgr/lib/dcmgr/configurations/natbox.rb:11:in `<class:Natbox>': uninitialized constant Dcmgr::Configurations::Natbox::Configuration (NameError)
        from /opt/axsh/wakame-vdc/dcmgr/lib/dcmgr/configurations/natbox.rb:9:in `<module:Configurations>'
        from /opt/axsh/wakame-vdc/dcmgr/lib/dcmgr/configurations/natbox.rb:7:in `<module:Dcmgr>'
        from /opt/axsh/wakame-vdc/dcmgr/lib/dcmgr/configurations/natbox.rb:5:in `<top (required)>'
        from ./bin/natbox:19:in `block in <main>'
        from ./bin/natbox:18:in `instance_eval'
        from ./bin/natbox:18:in `<main>'
```