consumer: handle problematic ZMQ->AMQP bridge message format
    
    The ZMQ->AMQP bridge (which republishes fedmsgs to the AMQP
    fedora-messaging broker) formats the messages it publishes in
    a way that likely won't match how equivalent messages will look
    once the publisher is converted to native AMQP publishing:
    
    https://github.com/fedora-infra/fedmsg-migration-tools/issues/20
    
    Specifically, it uses the *entire fedmsg* as the AMQP message
    body, whereas it should probably use only the 'msg' dict from
    the fedmsg as the body - that's the content that publishers are
    likely to provide as the message body when they convert to AMQP.
    Let's try and be smart about handling both possible formats here
    so this code will keep working as publishers are converted, and
    also if we 'fix' the bridge.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>