You can’t schedule opportunity. In baseball, smart planning, dumb good luck and maturing stars arrive on their own schedule. Mystery ingredients sometimes arrive as well, often in unexpected, almost paradoxical forms. That’s where the Washington Nationals find themselves as they arrive at training camp. They have the blossoming talent, as well as the offseason acquisitions to be one of the sport’s four preseason World Series favorites, bunched with the Blue Jays, Angels and Dodgers. But they also have the confluence of two other powerful motivational factors that can supercharge their season if things work out well, or seem twice as demoralizing if they don’t. They arrive here after one of the worst season-ending losses ever; and their respected, popular manager, Davey Johnson, 70, is retiring. If momentum builds during the season, atonement for the Nats and the fondest possible farewell for Johnson will drive them. If things go south, Oct. 12, 2012, looms larger and hell, Davey is leaving, too.