By Naomi Tajitsu and Maki Shiraki TOKYO (Reuters) - Top executives from Nissan and Renault on Thursday reaffirmed the importance of an alliance that has been pressured by the ousting of former Chairman Carlos Ghosn, but avoided discussing the potentially contentious issue of Nissan's next chairman. The newly appointed chairman of France's Renault SA, Jean-Dominique Senard, arrived in Japan on Thursday for two days of meetings with Nissan. Speculation has swirled about the future of the alliance, and whether Senard himself would assume the chairmanship of the Japanese automaker