ATLANTA — Two American aid workers who were infected with the deadly Ebola virus have been discharged from an Atlanta hospital, where the scene Thursday was festive and celebratory — a stark contrast to the sterile, rushed atmosphere that marked their arrival nearly three weeks ago. Dr. Kent Brantly, 33, and Nancy Writebol, 59, were infected while working at a missionary clinic in the West African nation of Liberia. They were given the experimental drug Zmapp and flown back to the United States for treatment.