Journalist Mary Lisa Gavenas’ new book details cosmetics titan Mary Kay’s rags-to-riches story. Born in Hot Wells, Tex., Mary Kay Ash overcame early hardship to build a billion-dollar empire. Her company, Beauty by Mary Kay, went public in 1968, a first for a woman-chaired firm on NYSE. A new biography of Mary Kay — the big-haired, bleach-blonde and perpetually pink-clad cosmetics titan — reads more like a juicy Judith Krantz novel.  In “ Selling Opportunity ” (Viking), journalist Mary Lisa