By Piya Sinha-Roy LOS ANGELES (Reuters) - As the summer festival season kicks off on Friday with Southern California's annual music event Coachella, the event has side-stepped the chart hitmakers to this year return to its rock music and taste-maker roots. While the Coachella Valley Arts and Music Festival in Indio, California, secured top-selling artists such as Paul McCartney, Prince, The Killers, Kings of Leon, Jay-Z and Kanye West in the past, this year's headliners include two recently reunited Brit-rock bands and a rising French band. ...