NEW YORK (Reuters) -Frontier Airlines CEO Barry Biffle said the ultra-low- cost carrier model is 'alive and well' in the United States amid claims from industry peers that the model is struggling. The Denver-based airline said it is committed to becoming the leading low-fare carrier in the top 20 U.S. metros and will help fill gaps left by Spirit Airlines, which filed for bankruptcy protection last month for the second time in a year. "You've got too much domestic supply and that is hurting yields across ...