By Christoph Steitz, Tom Käckenhoff, Edward Taylor and Arno Schuetze. : FRANKFURT, May 17 (Reuters) - When Thyssenkrupp CEO Guido Kerkhoff announced plans to list its prized elevators unit last week, he set off a battle for the conglomerate’s future that could test Germany’s brand of “social market” capitalism. Kerkhoff had little choice but to think the unthinkable when the company’s share price sank to a 15-year low on May 8. The plan to take elevators public not only broke with long tradition but marked the abandonment of a turnaround plan he himself had unveiled less than eight months before.