Fix template variable to use @-notation

This fix prevents the following messages:

    Variable access via 'collectd_version' is deprecated. Use '@collectd_version' instead. template[/etc/puppet/modules/collectd/templates/plugin/network/listener.conf.erb]:2
       (at /usr/lib/ruby/vendor_ruby/puppet/parser/templatewrapper.rb:76:in `method_missing')