By Carolyn Cohn and Wayne Cole LONDON (Reuters) – U.S. stock index futures rallied, the euro rose and global stocks steadied on Monday as a glimmer of hope emerged for a diplomatic solution to the Russia-Ukraine standoff. U.S. President Joe Biden and Russian President Vladimir Putin have agreed in principle to hold a summit on the Ukraine crisis. The Kremlin said there were no concrete plans in place for a summit, but that a call or meeting could be set up at any time. A summit would be held only if Russia did not first invade Ukraine, which Western countries have said it could do at any momen…