Support a list of topics

This change allows consumers to specify a list of topics in their consumer, like so:

```python
class OSTreeComposer(fedmsg.consumers.FedmsgConsumer):                
    topic = ['org.fedoraproject.prod.bodhi.updates.fedora.sync',      
             'org.fedoraproject.prod.compose.branched.rsync.complete',
             'org.fedoraproject.prod.compose.rawhide.rsync.complete'] 
    …
```