WASHINGTON (Reuters) – President Barack Obama celebrated his 51st birthday on Saturday with a round of golf, then was to head to the Camp David presidential retreat for a quiet evening away from the campaign trail. Obama, an avid golfer, played at Andrews Air Force Base. The White House did not release details of the president’s evening plans, or the menu for the birthday meal he was expected to share with wife, Michelle, at the retreat in Maryland’s Catoctin Mountains. …