Fix upgrade (update_replica_config) in single master mode
    
    Commit afc0d4b62d043cd568ce87400f60e8fa8273495f added an upgrade
    step that add an attribute to a replica config entry.  The entry
    only exists after a replica has been added, so upgrade was broken
    for standalone server.  Catch and suppress the NotFound error.
    
    Related to: https://pagure.io/freeipa/issue/7488
    
    Reviewed-By: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Fraser Tweedale <ftweedal@redhat.com>
    Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>