Brandy is consumed by wealthy Mexicans and the not so rich as well while they sit on bar stools or gossip in hotel lobbies or talk across a table in an outdoor cafe or watch the waves on a beach in Puerto Vallerta. The most popular brandy is Domecq's Presidente, so widely drunk - over five million cases last year - that the brand is virtually a generic name for brandy here. A light, easily mixed spirit, it has been aged only two years and is not likely to be confused with heady, decades-old spirits. Mexicans most often drink it mixed with Coca-Cola, water, ginger ale, soda water or 7-Up. By Gerald Eskenazi Los Reyes, Mexico Sept. 5, 1984