By James Davey LONDON (Reuters) – Steve Rowe will step down as chief executive of Marks & Spencer in May after six years in the job, with the British retailer opting for continuity by choosing two of his senior lieutenants as his successor. Rowe, who joined M&S at the age of 15 before rising through the ranks, will step down after the food and clothing retailer reports annual results on May 25. His tenure in one of the most high-profile jobs in British business has been turbulent as he, along with chairman Archie Norman, has sought to transform M&S by improving the quality and value of its clo…