HILLSBORO, Oregon (Reuters) - Democratic presidential candidate Hillary Clinton found herself on the defensive over a top aide's meeting with Colombia on a trade deal, as she and rival Barack Obama courted voters on Saturday in Western states. Mark Penn, chief campaign strategist to the New York senator, has apologized for meeting with Colombia's U.S. ambassador in his separate role as a lobbyist hired by the South American country to win congressional approval of the trade deal with the United States. But the issue poses problems for Clinton for political reasons as well as diplomatic ones as she vies with...