Allow bundle to override default_locale

... in case `kernel.default_locale` cannot be used.

I was unable to fork the documentation, seems to be broken?
$config['default_locale'] exist with null value in A2lixTranslationFormExtension when you don't declare it explicitly?
Or throws a PHP Notice exception because the key doesn't exist?

Otherwise, I'll be more for ````->scalarNode('default_locale')->defaultValue('')->end()````