David Nahai is an environmentalist and an attorney, not an engineer, and his major previous management challenge was running a 15-employee law firm. But he is the man Los Angeles Mayor Antonio Villaraigosa tapped take on the $304,000-a-year job as general manager of the Department of Water and Power, the nation\’s largest — and frequently troubled — public municipal utility. He\’s also the first ever to helm the DWP without decades of experience in either the utility business or city government.