fix: BROKER_URL to honor REDIS_PORT and REDIS_DB
    
    Config Parameter BROKER_URL (https://docs.pagure.org/pagure/configuration.html#broker-url)
    is used by Celery workers and doesn't honor REDIS_PORT and REDIS_DB by default,
    it's right now expected that the User overwrite the BROKER_URL parameter if
    a non-standard port / db is required.
    
    Both parameters are defined in default_config.py, also based on Celery Docs:
    "all fields after the scheme are optional, and will default to localhost on port
    6379, using database 0".
    
    Therefore it's safe and more user friendly to change the default of BROKER_URL to:
    "redis://%s:%d/%d" % (pagure_config["REDIS_HOST"], pagure_config["REDIS_PORT"], pagure_config["REDIS_DB"])