WASHINGTON, March 6 (Reuters) - The United States is well on its way toward controlling Iranian airspace, White House press secretary Karoline Leavitt said on Friday, adding that Washington expects the achievable U.S. objectives to be completed in four to six weeks.Speaking to reporters at the White House, Leavitt also said Washington was looking at potential candidates to lead Iran, a day after President Donald Trump told Reuters in an interview that the United States must be involved in choosi...