use collectd.conf.d
Double manage the conf.d dir as workaround

Salt annoyingly reports changes to files in directories if marked
as 'clean: True'. This is a hack to prevent that, but still
maintain a clean directory.

See sensu-formula for more history.
update CHANGELOG
Merge pull request #25 from ministryofjustice/manage_conf_d

Manage /etc/collectd.conf.d dir