By Amina Niasse and Sriparna Roy NEW YORK (Reuters) -Some long-term UnitedHealth Group investors are putting their faith in the return of a CEO with a proven track record and a new management team they believe can deliver a turnaround for the historically high-growth Optum health services business and put a tough year behind it. Investors, billionaire Warren Buffett's Berkshire Hathaway among them, have reacted favorably to the selection of Stephen Hemsley, who took over in May after the company missed i ...