By Mark Lamport-Stokes OAKMONT, Pennsylvania, June 15 (Reuters) - Twelve months after his U.S. Open dream became a nightmare with a three-putt on the final hole at Chambers Bay, Dustin Johnson was in upbeat mood on the eve of this year's edition at Oakmont Country Club. The athletic American has a power game that makes him capable of winning on any golf course and he will tee off in Thursday's opening round buoyed by top-five finishes in his last two PGA Tour starts. More...