By Elena Rodriguez and Michael Francis Gore MADRID (Reuters) – Carrying the hammer and sickle flags of the former Soviet Union, thousands protested in Madrid on Sunday against a NATO summit which will take place in the Spanish capital next week. Amid tight security, leaders of the member countries will meet in Madrid between 29-30 June as the organisation faces the unprecedented challenge of the Russian invasion of Ukraine. NATO is expected to consider the bid, opposed by alliance-member Turkey, for Finland and Sweden to join. The Nordic nations applied in the wake of the Russian assault on Uk…