She has been famous for more than seven decades " and has as much zest and ambition today as she did at 18. She discusses fame, politics, the casting couch, motherhood and marrying five times Joan Collins steps out of the chauffeur-driven limo " white frock, wide-brimmed white hat, huge shades, rocks on her fingers. Even if she wasn't Joan Collins, you would stop to look. Some people just have the air of a star. At 9…