The board may enter into contracts or agreements with any persons, firms or corporations for the operation and management of the public service properties within the district, or any part thereof, for such period of time and under such terms and conditions as shall be agreed upon between the board and such persons, firms or corporations. The board shall have power to provide in the resolution authorizing the issuance of bonds, or in any trust indenture securing such bonds, that such contracts or agreements shall be valid and binding upon the district as long as any of said bonds, or interest thereon, are outstanding and unpaid.