The Clinton administration may be preoccupied with Cuba, but this week it will send an unmistakable signal to Haiti's military leaders. Deputy Secretary of State Strobe Talbott and Deputy Secretary of Defense John Deutch plan to fly to Jamaica Tuesday to meet with Caribbean leaders. Their mission: to secure aid for a possible Haiti invasion and a post-invasion peacekeeping force. The administration initially planned to send a low-level delegation, but it settled on Talbott and Deutch to make sure the Haitians get the message. Pentagon officials insist they have enough ships and manpower to invade Haiti in the midst of the Cuban crisis. But officials say they probably won't act until the United Nations gives Haiti's generals a final warning.