Section 21. The colonel may conduct programs for training police officers of the cities and towns of the commonwealth and for those employees who are authorized to exercise police powers in the various state departments or agencies and the colonel shall appoint necessary instructors. Said programs shall be conducted and instructors appointed in accordance with the provisions of section one hundred and eighteen of chapter six. Cities, towns, state departments and agencies voluntarily participating in such training shall reimburse the commonwealth for the necessary expense thereof. Attendance at such programs by police officers of any city or town and those employees who are authorized to exercise police powers in the various state departments or agencies shall be on a voluntary basis.