All staff officers of the Kansas national guard, including officers of the pay, inspection, subsistence, and medical departments, hereafter appointed shall have had previous military experience, and shall hold their positions until they shall have reached the age of sixty-four years, unless retired prior to that time by reason of resignation, disability, or for cause to be determined by a court-martial legally convened for that purpose; and all vacancies among said officers shall be filled by appointment from the officers of the militia of Kansas. Nothing in this section shall be construed or operate to limit the provisions of any federal law relative to the qualifications for appointment of officers or the filling of vacancies.