celery: addition of Flask support

• Adds Flask support to Celery by ensuring that the Flask application and request context is set so tasks can make use of e.g. SQLAlchemy.

• Changes default result/task serializer from json to msgpack (requires Python 2.6+ due to msgpack).

• Changes default result store from RabbitMQ to Redis.

Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>