MIAMI (Reuters) - While hundreds of U.S. law enforcement agents intercepted imaginary Cuban migrants during a massive training exercise in south Florida, two boatloads of actual Cubans sneaked ashore on Miami Beach on Thursday. Boaters dropped off 21 Cuban migrants at a popular nudist beach and left 19 others on another beach a few hours later, the Border Patrol said. Both vessels escaped. "It's our belief that they were the result of organized smuggling," Border Patrol spokesman Steve McDonald said. The Cubans arrived on day two of a training exercise to test "Operation Vigilant Sentry," the U.S. Department of Homeland...