Every contractor, subcontractor or city officer performing any work for the city upon streets, public places or public property, shall pay or cause to be paid to his employees on such work not less than the current rate of wages paid for work of like character; said contractor and subcontractor shall on such work give preference to resident laborers, honorably discharged war veterans and citizens of the United States who are heads of families. This article shall be enforced by the City Council, and the violation of any of the provisions of this section by any contractor or subcontractor shall be sufficient ground for forfeiture of his contract.