NEW YORK (Reuters) - A leading restaurant association has called for the cancellation of a TV commercial featuring Britney Spears' estranged husband, Kevin Federline, as a failed rap star working in a fast-food eatery. In a 30-second ad for Nationwide Insurance, Federline is shown dreaming he is a rap star but then snaps out of it to face reality -- he's working at a burger restaurant. The commercial is due to be aired during the National Football League's Super Bowl championship on Sunday, February 4, advertising's biggest televised sporting event of the year. Last year's Super Bowl drew more than...