Ensure config file is absent when service is declared absent

If `Supervisor::Service['fooservice']` had previously been declared, but now we want to delete the associated supervisor configuration, we should be able to do something like this:

    supervisor::service { 'fooservice':
        ensure => 'absent',
        command => "/usr/bin/foo",
    }

However, this would not delete the configuration file `/etc/supervisor/fooservice.init`. This PR fixes that.