A Study on the Background of the Reduction of the Tianbing army(天兵軍) and the Establishment of Taiyuan Fu(太原府) in the 11th Year of the Kaiyuan Era(開元) : Focusing on the Changes in the Relationship with the Türks(突厥) from the Wu Zetian(武則天) to the Xuanzong(玄宗) Era