EXCLUSIVE: Geoff Stults has been added to the cast of the Nicolai Fuglsig-helmed drama Horse Soldiers. He joins Chris Hemsworth, Michael Shannon, Michael Pena, and Austin Stowell in the pic based on the true story about a U.S. Special Forces Team and their untested captain sent to a rugged region of Afghanistan in the weeks following September 11th. Stults will play Sean Coffers, the team’s complainer, constantly cynical about the mission, but does his job well. The…