Allow fedmsg.meta plugins to hardcode topic prefix.

But only if they choose to do so.  This should be backwards compatible
where the topic_prefix_re comes from the /etc/fedmsg.d/ config.

This is a fix for fedora-infra/fedmsg_meta_fedora_infrastructure#135.
Merge pull request #280 from fedora-infra/feature/topic-prefix-hardcode

Allow fedmsg.meta plugins to hardcode topic prefix.