“ There is hereby delegated to the Secretary of State the authority vested in the President by Section 102 (a) of the Act (5 U.S.C. 5702 (a) ) to establish maximum rates of per diem allowances and reimbursements for the actual and necessary expenses of official travel for employees of the Government to the extent that such authority pertains to travel status in localities (including the Trust Territories of the Pacific Islands) in any area situated outside the United States, the Commonwealth of Puerto Rico, and the possessions of the United States. ”