Kafka support for filebeat, topbeat and packetbeat

[Tilman Baumann]

I just added Kafka output to filebeat, topbeat and packetbeat.
If anyone is interested...

"charm test" is broken for me, apologies if I committed a broken test.
I manually tested the filebeat changes, the rest is just copy-pasta.
(same code everywhere)

https://github.com/juju-solutions/layer-beats-base/pull/15
https://github.com/juju-solutions/layer-filebeat/pull/19
https://github.com/juju-solutions/layer-topbeat/pull/13
https://github.com/juju-solutions/layer-packetbeat/pull/5


Cheers
 Tilman Baumann