Rename `seconds-to-live` to `seconds_to_live` (but keep backward compatibility).
    
    All API arguments use underscore except of `seconds-to-live`. In this commit,
    this is change to make the API more consistent.
    
    It also makes it easier to use `**kwargs` in methods using the API, because
    you cannot use `"-"` character in the name of `kwarg`.
    
    Signed-off-by: Jan Kaluza <jkaluza@redhat.com>