By Gabriella Borter, Tim Reid and Nathan Layne DES MOINES, Iowa (Reuters) -Nikki Haley has overtaken Ron DeSantis in Iowa in a closely watched poll just two days before a first-in-the nation presidential nominating contest will determine whether either Republican can emerge as a viable alternative to Donald Trump. Trump remains the dominant candidate in Iowa and the favorite to win their party’s nomination and take on U.S. President Joe Biden in what is expected to be a close and deeply acrimonious November vote that has raised questions about the depth of support for Europe and even basic dem…