[DX] Re-work generate:bundle for simple app bundles #290

@weaverryan please review my pull request ;)
I think we should follow the `with-repository` option example in this command: https://github.com/sensiolabs/SensioGeneratorBundle/blob/master/Command/GenerateDoctrineEntityCommand.php#L162

So:

- make this InputOption::VALUE_NONE
- make it default to false

Then, we can remove the validator function that we had to add. It also avoids needing to cover all cases - like `--shared=true`, `--shared=y`, `--shared=yes`, etc. It's simply `--shared` or not :).