Section 50. No person habitually using intoxicating liquors to excess shall be appointed to or employed or retained in any civil service position, nor shall any person be appointed to or employed in any such position within one year after his conviction of any crime except that the appointing authority may, in its discretion, appoint or employ within such one-year period a person convicted of any of the following offenses: a violation of any provision of chapter ninety relating to motor vehicles which constitutes a misdemeanor or, any other offense for which the sole punishment imposed was (a) a fine of not more than one hundred dollars, (b) a sentence of imprisonment in a jail or house of correction for less than six months, with or without such fine, or (c) a sentence to any other penal institution under which the actual time served was less than six months, with or without such fine. Violations of statutes, ordinances, rules or regulations regulating the parking of motor vehicles shall not constitute offenses for purposes of this section.