dynamic config support

[glensc]

spew from #428

i wish to listen to topic, but not to hardcode it to code, because the prefix may differ.

i solved this in my project this way:

    def __init__(self, hub):
        # I'm only interested in messages from CVS
        self.topic = self.abs_topic(hub.config, "cvs.commit")

        super(CVS2SlackConsumer, self).__init__(hub)

    # no proper way to configure just topic suffix
    # https://github.com/fedora-infra/fedmsg/pull/428
    def abs_topic(self, config, topic):
        """
        prefix topic with topic_prefix and environment config values
        """
        topic_prefix = config.get('topic_prefix')
        environment = config.get('environment')
        return "%s.%s.%s" % (topic_prefix, environment, topic)

please provide official support for such approach, perhaps just implement abs_topic() fedmsg.consumers.FedmsgConsumer until new config system is in place.

i'm duplicating this logic in consumers i write for now, which is just bad practice.

[glensc]

other idea is to add self.topic_suffix aside self.topic which __init__ can handle the way above code does.

[jeremycline]

This makes sense to me. A pattern I've seen through-out Fedora's consumers is a "Am I in dev/stg/prod? search-and-replace the topic I listen to" approach which is not very pleasant.