If Soviet-American relations are cool in the aftermath of the South Korean airliner tragedy, they are especially chilly in Kansas City. Last year civic leaders cheerfully cooperated with a Soviet film crew producing what they were told was a TV documentary about life in Middle America. Now they are embarrassed to find out they played roles in a propaganda film. ''They showed people wandering in the streets and sleeping in the park,'' said Mayor Richard Berkley. ''There were scenes shown that they represented as police brutality.'' The film showed commodity futures traders at the Kansas City Board of Trade, describing them as ''greedy capitalist beetles.''