WASHINGTON (Reuters) - Nigerian Finance Minister Ngozi Okonjo-Iweala and former Colombian Finance Minister Jose Antonio Ocampo are set to be nominated to lead the World Bank, sources with knowledge of emerging market efforts to find candidates said on Tuesday. The candidacies of Okonjo-Iweala and Ocampo, who have credentials as both economists and diplomats and according to sources the respective backing of Brazil and South Africa, pose a challenge to the United States, whose hold on the top post has never been contested. ...