global: not possible to overwrite celery.flask_app

Because of the check that's done [here](https://github.com/inveniosoftware/flask-celeryext/blob/master/flask_celeryext/app.py#L59-L60) for the existence of the `flask_app` attr on the celery instance, it's not possible to explicitly set the `flask_app` after the first initialization of an application.

This affects mainly tests, since when this is initally set to some app (with config X) the first time at some test fixture, it's impossible to overwrite it with a different app (with config Y).