A certain amount of celebrity glow has long been a part of both political parties’ gatherings. But thanks in part to the youthful charisma of Senator Barack Obama, the presumptive nominee, the Democratic convention, which begins on Monday in Denver, is shaping up as an unlikely hot spot for the music world, with multiplatinum rappers, indie-rock scenesters, D.J.’s and Jennifer Lopez arriving by the van- and private planeload to perform, rally or schmooze with the political elite. “It’s the Sundance Film Festival for politicos,” said Laura Dawn, the cultural director of MoveOn.org, who also happens to sing with Moby... Kanye...