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?
Yes I well understood, but as I doubted, ````->scalarNode('default_locale')->isRequired(false)->end()```` raise an exception for people with minimal configuration:

````
a2lix_translation_form:
    locales: [en, fr, de, es]
    required_locales: [en]
````

So update your code with ````->scalarNode('default_locale')->defaultValue('')->end()````