By Nathan Layne, Gabriella Borter and Tim Reid INDIANOLA, Iowa (Reuters) -Republican presidential candidates exhorted their supporters to brave glacial, life-threatening temperatures in Iowa to vote in the party’s first nominating contest on Monday, amid fears a predicted record cold snap could keep many voters at home. The icy weather in the Midwestern state has become a wild card in the crucial contest that could help seal former President Donald Trump’s bid to become the Republican nominee to face Democratic President Joe Biden in November’s general election. His top rivals, former U.N. amb…