By Max Hunder and Tom Balmforth KYIV (Reuters) -Russian forces abandoned the strategic Black Sea outpost of Snake Island on Thursday, in a major victory for Ukraine that could loosen the grip of Russia’s blockade on Ukrainian ports. Russia’s defence ministry said it had decided to withdraw from the outcrop as a “gesture of goodwill” to show Moscow was not obstructing U.N. efforts to open a humanitarian corridor allowing grains to be shipped from Ukraine. Ukraine said it had driven the Russian forces out after a massive artillery and missile assault overnight. “KABOOM!” tweeted Andriy Yermak, U…