Purge configs

this optionally supports purging the supervisor.d directory to remove old services that are no longer being placed by puppet.

I'm a little concerned about the deps though. I had to remove the blanket `supervisord::config ~> supervisord::service` so it wouldn't try to restart the whole service when files get removed, and in theory only changing the defaults or init file will cause a full restart.

I'm not sure how to write a test to cover that scenario.