Actor David Keith, who has played military-minded characters in "The Lords of Discipline" and "An Officer and a Gentleman," is about to play the ultimate Marine - Oliver North. Keith left Tuesday for Los Angeles after a vacation in Aspen, Colo., to start work next week on a television mini-series about North, who Keith describes as "a brave man, a real cowboy." Keith says he landed the part only after he was able to convince producers he could lose his Tennessee accent. The mini-series, based on the biography "Guts and Glory" by Boston Globe reporter Ben Bradlee Jr., will begin with North's days at the Naval Academy and follow him until his retirement last year in the midst the Iran-Contra affair.