Preheat oven to 400. Prepare a 11"x7" casserole dish with cooking spray; set aside. In a large saucepan, cook chicken until no longer pink. Add taco seasoning mix, tomatoes, water, macaroni, and celery. Bring mixture to a boil; reduce heat to low and cover. Cook 20 minutes, or until macaroni is done. Spoon chicken mixture into prepared pan; set aside. Meanwhile, combine cornbread mix, egg, and milk. Mix well. Spread cornbread mixture evenly over top chicken mixture. Bake for 20 minutes, or until cornbread topping has browned.