Define start fedmsg json value
    
    The nightly scripts try to send an announcement about compose starting.
    As this happens before Pungi even started, the compose ID is not known
    and can not be included in the message. The message about starting rsync
    sent later will contain the ID correctly.
    
    Without the json defined there is a traceback in the cron output, but
    the errors are not logged anywhere else (but don't actually block
    anything in the compose process).
    
    Relates: https://pagure.io/pungi/issue/698
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>