PHILADELPHIA, Aug 26 (Reuters) - Three years before retiring from the U.S. Army in 2017, Donald Trump-backed Pennsylvania gubernatorial candidate Doug Mastriano posed in Confederate uniform for a faculty photo at the Army War College, according to a copy of the photo obtained by Reuters. The previously unreported photo, released by the War College to Reuters after a request under the Freedom of Information Act, showed Mastriano in a 2013-14 portrait for the Department of Military Strategy, Plans, and Operations, where he worked. Faculty at the time had been given the option of dressing as a historical figure, people familiar...