Support for client.dns.lookup=resolve_canonical_bootstrap_servers_only

# What is currently missing?

Better support when kafka is running in kubernetes.

When setting up kafka in kubernetes, one can setup the client's bootstrap_uri to point to a kafka headless service and take advantages of k8s's dns to get back a list of canonical kafka broker pod names (FQDNs) that can then be used by the client to establish a proper broker bootstrap connection.

In order for that to work when the kafka brokers are setup with SSL or SASL_SSL, kafka clients must set client.dns.lookup=resolve_canonical_bootstrap_servers_only. This is because proper ssl support requires clients to connect to the broker using an FQDN and the cert returned by the broker will need to have a subjectAltName field that can match the FQDN).

The suggestion is to add a setting that functions like client.dns.lookup. See https://cwiki.apache.org/confluence/display/KAFKA/KIP-602%3A+Change+default+value+for+client.dns.lookup