alter default FormFlow storage using parent service attribute

services.yml

services:
    ormigo.form.flow:
        class: Craue\FormFlowBundle\Form\FormFlow
        parent: craue.form.flow
        scope: request
        calls:
            # see https://gist.github.com/3066310
            - [ setStorage, [ @ormigo.form.flow.storage.mongodb ] ]

    ormigo.form.image_upload.flow:
        class: Ormigo\Bundle\OrmigoBundle\Form\ImageUpload\FormFlow
        parent: ormigo.form.flow
        scope: request
        arguments:
            - @liip_imagine
        calls:
            - [ setFormType, [ @ormigo.form.image_upload.form ] ]