Added required attributes for cmd parser mixins after changes in Salt 0.15.9

Without these changes, salt-api will not start when installed with Salt 0.15.9 (dies with an error saying _default_logging_logfile_ is not defined).

See Salt commits c3bfde339a and 92da051c00. Also moved the default logging location to /var/log/salt to bring it inline with the rest of the Salt logs.