Add optional fedmsg notification for secondary
    
    To reduce the I/O load during master mirror scanning an appropriate sync
    finished fedmsg is now emitted at the end of each rsync run.
    
    Only the rsync part has been tested stand-alone and then integrated into
    the releng secondary-sync.py script.
    
    v2:
       * changed fedmsg from
         "compose.updates.rsync.complete" to
         "releng.secondary.updates.rsync.complete"
       * pass transferred and deleted both as strings
    
    Signed-off-by: Adrian Reber <adrian@lisas.de>