By Steve Holland and Roberta Rampton WASHINGTON (Reuters) – President Barack Obama imposed the most sweeping immigration reform in a generation on Thursday, easing the threat of deportation for some 4.7 million undocumented immigrants and setting up a clash with Republicans who vow to fight his moves. In a White House speech, Obama rejected Republican arguments that his decision to bypass Congress and take executive action was tantamount to amnesty for illegal immigrants. …