By Marianna Parraga HOUSTON (Reuters) -A U.S. judge allowed an affiliate of hedge fund Elliott Investment Management on Thursday to move towards completing its offer for Venezuela-owned Citgo Petroleum's parent through a court-ordered auction of shares, while restricting rival bidder Gold Reserve from doing the same. This followed a separate judge in another U.S. court upholding the validity of Venezuelan state oil company PDVSA's 2020 bonds on Thursday. Delaware Judge Leonard Stark instructed a court o ...