By Alexei Anishchuk MOSCOW (Reuters) - Former Soviet leader Mikhail Gorbachev called on Wednesday for a U.S.-Russia summit to be convened to prevent a deep freeze in Moscow's relations with the West over the Ukraine crisis. Gorbachev, whose policy of "perestroika" (restructuring) played a role in ending the Cold War, warned of potentially dire consequences if tensions were not reduced. "This is extremely dangerous, with tensions as high as they are now. ...