(Adds details) PARIS, Oct 18 (Reuters) - Spint ace Mark Cavendish is leaving Team Sky in order to fulfil his Tour de France ambitions after spending this year's race serving as a domestique for race winner Bradley Wiggins, the British team said on Thursday. Briton Cavendish, the 2011 world champion who has 23 Tour de France stage wins, will join Belgian team Omega Pharma-Quick Step for the 2013 season. Sky's Team Principal Dave Brailsford said Cavendish had been instrumental in the relatively new outfit's success. ... More...