By Mathieu Rosemain and Julien Ponthus PARIS (Reuters) -The French government holds the final keys to any move to buy the BDS cybersecurity division of France’s Atos, industry sources and analysts said, after French defence company Thales was reported to be devising a break-up plan. Atos said on Thursday that its cybersecurity arm, BDS, was not for sale after sources told Reuters that Thales was working on a plan to buy the business, having approached private equity firms about exploring a joint offer and break-up. Atos, formed partly by a string of acquisitions made under former CEO Thierry B…