Improve Varnish plugin

- 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.
Merge pull request #171 from alphagov/improve_collectd_varnish

Improve collectd varnish