By John Irish and Sam TabahritiPARIS/LONDON, April 1 (Reuters) - Tensions between the United States and NATO allies flared up as President Donald Trump said he was considering pulling the U.S. out of the military alliance due to its European members' refusing to send ships to unblock the Strait of Hormuz.NATO, which includes European countries, the United States and Canada, was formed in 1949 with the aim of countering the risk of Soviet attack and has been the cornerstone of the West's security...