Switch to Filebeat 6.x for docker log type

[Shane Peters]

Filebeat 6.x provides a new log type called 'docker'. Ideal for kube-logs, this significantly helps decoding multi-line JSON messages. More specifically, java exceptions, which at this point generate multiple log messages for a single stack trace.

I've tested this using the cdk-monitoring.yaml bundle, upgrading filebeat to 6.x, and configuring the new docker log type (after disabling jujud-unit-filebeat).

Using an example java exception encoded in json at https://pastebin.ubuntu.com/p/wqjwrNDFBv/ and appending it to an existing docker log, I'm able to see a single log message in graylog for it. Before, this would have been separated into 4 different messsages.