At the end of the first installment of Francesca Zambello’s American Ring Cycle, last year’s Das Rheingold premiered at Washington National Opera, the gods went into Valhalla on what looked like the gang plank of a cruise liner, clinking their champagne flutes. Richard Wagner adapted the libretti of his four-opera cycle from German mythology, and Zambello’s idea was to exchange the German myths in the operas for American ones. The gold-hungry Alberich became a…