Fix message format for "container-image.test.complete" topic
    
    Validated in: https://pagure.io/fedora-ci/messages
    
    Changes value for field "environment" to "stage" - it can be either
    "stage" or "production".
    
    Adds required field "generated_at" - contains date/time in ISO 8601
    format.
    
    Signed-off-by: Lukas Holecek <hluk@email.cz>