Update kafka_package_name to confluent-kafka-2.12

In https://github.com/jcustenborder/puppet-confluent/blob/9bc2da5441d5ff0e5f2c10a3e4c8dc6b6191b099/manifests/params.pp#L174, it hardcodes a specific (outdated) version of the confluent-kafka package. We suggest that it should be updated to 2.12. I am not sure why the version number is embedded into the package name, as that makes maintenance a pain, but whatever :)

~tommy