Support kafka output testing

[Popsiclestick]

Describe the enhancement:
Beats Kafka output does not currently support output testing. We should add functionality for verifying connectivity of the kafka output.

Describe a specific use case for the enhancement or feature:
This would allow users to test their beats ability to interface with kafka before starting their application. Kafka output is a common use case for beats and this would be a helpful test/healthcheck.

Current behavior:

:; ./filebeat test output
kafka output doesn't support testing

[jsoriano]

Closing this, and created #10652 for TLS support on this check.