With the advent of Kafka 0.9.0+ it is possible to set up TLS client
certificate based authentication to limit access to Kafka.
Four new configuration variables are specified for setting up the
authentication. If they're not set the behavior stays the same as
before the change.
closes#541
Cameron Sparr
Stephen Kwong
Jack Zampolin
Hannu Valtonen
Eugene Dementiev
Thibault Cohen