Preserve property order in models.json

388847 added `public` as a property to `ModelConfig`, which had an unintended side-effect of changing the order of properties in `models.json`. Before that change, `dataSource` came before `public`. After that change, `public` is first.

This commit fixes the problem by explicitly defining `dataSource` property in ModelConfig definition. (The property is otherwise added by "belongsTo DataSource" relation.)

See also https://github.com/strongloop/generator-loopback/issues/25.

/to @ritch please review