Whenever any municipality shall issue any bonds or other evidence of indebtedness which are payable solely from revenues to be derived from any system, the governing authorities of such municipality may, by appropriate provision in the ordinance or resolution authorizing the issuance of such bonds, or by separate resolution or ordinance passed at or prior to the actual sale of such bonds, bind and obligate such municipality to take, for a period not exceeding the full term of such bonds, at least a stated minimum of the services to be afforded by such system and to pay, out of its corporate funds, a least a stated minimum price therefor. Such provision, resolution or ordinance shall constitute a contract between such municipality and all the holders of such bonds.
All such agreements heretofore entered into by any such municipality, whether such bonds have actually been delivered and paid for or not, are hereby ratified, approved and validated.