By Joseph White (Reuters) – Herbert Diess took over at Volkswagen AG four years ago when the German automaker was in crisis, under pressure to make dramatic changes in strategy and culture, and the former BMW executive offered a new vision. Diess will leave Volkswagen on Sept. 1, three years before his contract was supposed to end, with many of the goals he set as the German auto giant’s disruptor-in-chief unfinished and uncertain. Prominent among them, Diess’s effort to create a Germany-centered software business, CARIAD, within Volkswagen, and a promised stock market listing for the Porsche …