By Andrew GrayBRUSSELS, March 5 (Reuters) - NATO Secretary General Mark Rutte on Thursday welcomed French President Emmanuel Macron's pledge to expand his country's nuclear deterrent but said the U.S. nuclear umbrella would remain the ultimate guarantor of European security.Macron this week announced France would increase its nuclear arsenal and potentially allow European partners to host French warplanes on deterrence missions, following concerns among allies about the reliability of the U.S..