Seafood abounds at the Plaquemines Parish Seafood Festival.Photos: Emily PetersonThe sixth annual Plaquemines Parish Seafood Festival, held this past weekend, had the usual fixings one would expect at a South Louisiana festival: fried seafood, a solid lineup of live local music, and plenty of cold beer to beat the high humidity and 90-degree temperatures. One element was notably absent: an apocalyptic focus on the oil spill.    Plaquemines Parish is ground zero in what is now considered "the biggest environmental disaster we've ever faced in this country," as White House energy and climate change adviser Carol Browner put it on …