remote_cube_create now accepts username/password

Small refactor also to send all the cube attributes to
remote_cube_create.

No we should be able to do:

```
cube = octoc.cubes.create :name     => name,
                          :source   => File.expand_path(file),
                          :username => 'user',
                          :password => 'secret'
```

And set cube's default username/password.