LONDON (Reuters) – Volkswagen’s Herbert Diess is stepping down as chief executive and will be replaced by Oliver Blume, who joined Europe’s biggest carmaker as a graduate trainee almost 30 years ago, the German carmaker said on Friday. Blume, 54, was considered one of the potential candidates to succeed Diess, whose frail relationship with German trade unions had created uncertainty about his future leadership. Here’s what we know about Blume’s career: * After studying mechanical engineering at university, he started his career in 1994 when he joined Audi’s international trainee programme. He …