Store compose_id for each event
    
    When each event ErrataAdvisoryRPMsSignedEvent happens, a series of
    docker images would be rebuilt with updated (newer version) RPMs.
    Each event will have its own compose (the YUM repository) that contains
    those updated RPMs, which will be used to rebuild docker images.
    
    When freshmaker gets message when a compose is generated successfully
    in ODCS, compose id will be used to retrieve docker images from
    database and start to rebuild.
    
    Signed-off-by: Chenxiong Qi <cqi@redhat.com>