Rob Kerth has been getting plenty of Zamboni action the last few nights. Midtown Business Association's executive director has been staying up late and going out in the early-morning dark to use the ice groomer to help create an ice-skating rink that opens at 10 a.m. Friday in Midtown. "It's a tremendous amount of work to put up one of these portable rinks," Kerth said Wednesday. "I've been there until 4 in the morning at least five out of the last 10 days." While most business association leaders might not even know how to spell "Zamboni," Kerth not only knows how to drive one, he owns one. And he has the license to drive it. That's because until last year, Kerth owned Ice Unlimited,