Once upon a time rock 'n' roll and soft drinks were two entirely different concepts but no longer. Rod Stewart is the latest to become a soda pop pitchman, having signed with Pepsi to sponsor the tour of South America and Mexico that he starts Feb. 26. In return, Stewart also will do commercials that will air throughout Latin America. Others who have done commercials for Pepsi or Coke include Tina Turner, David Bowie, Glenn Frey, George Michael, Michael Jackson and Don Johnson.