“Rocketman,” the life story of Sir Elton Hercules John, born Reginald Dwight, is an old-style movie musical that juxtaposes the spangly world of a 1970s rock star — all bright lights, glitter and razzmatazz — against the solitary existence of a pudgy piano prodigy, pounding away on a parlor upright in a drab inconsequential corner of postwar London.Both are imprisoned.Last year’s “Bohemian …