By Daniel Trotta (Reuters) -The National Rifle Association board reappointed Wayne LaPierre as executive vice president on Monday, turning back the latest leadership challenge amid corruption allegations and flat membership for the still-powerful gun lobby. The board vote came as the NRA held its annual meeting in Houston, about 280 miles (450 km) east of the site of a mass shooting on Tuesday, when an 18-year-old armed with an AR-15-style semiautomatic rifle killed 19 children and two teachers at a Texas elementary school. Conservative activist Allen West, a retired military officer and forme…