as many as 20,000 people visited a chapel every day and each visitor adds to the invisible cloud of dust, dirt, dandruff and other microscopic debris that floats to the ceiling. tribute bands are a staple of rock music. now they re making fans into rock stars in their own right while turning a profit. jane wells has more. on a summer afternoon in l.a., a park is packed with zeppelin fans. but this isn t led zeppelin. it s led zeppigan, a tribute band. it s in an era when some big names still won t put their music on itunes, tribute bands