Improve collectd varnish

- Fixes bug of being unable to use module if you have no current version of collectd - restriction to version 5.4 would block installation when version is nil
- Fixes bug of ignoring configs set to false - some of these will default to true if they are not explicitly set to false.
- More permissive about collectd version number - the user is expected to know what config options are allowed in their version.