To provide for the issuing of bonds, and for the application of bonds already issued, for the purpose of funding any and all indebtedness now existing or hereafter created of the city, now due or to become due. Said bonds shall be payable within thirty years from the time of their issue, and they shall be made payable in annual installments equal to the tax levied therefor, and shall be paid annually from the funds so provided. Said bonds shall bear interest, payable annually or semi-annually, at a rate not exceeding six per centum per annum, and shall be either registered bonds or coupon bonds with interest coupons attached. Said bonds shall not be sold or exchanged for less than their par value and accrued interest.