Add Shorewall module for Filebeat

[mdelapenya]

I'd like to contribute a filebeat module for Shorewall (a Linux firewall), which follows Net Filter pattern for logging messages (Please see http://logi.cc/en/2010/07/netfilter-log-format/)

Metricbeat Module / Dataset release checklist

This checklist is intended for Devs which create or update a module to make sure modules are consistent.

Modules

For a metricset to go GA, the following criterias should be met:

• Supported versions are documented
• Supported operating systems are documented (if applicable)
• Integration tests exist
• System tests exist
• Automated checks that all fields are documented
• Documentation
• Fields follow ECS and naming conventions
• Dashboards exists (if applicable)
• Kibana Home Tutorial (if applicable)
  • Open issue in EUI repo to add icon for module if not already exists.
  • Open PR against Kibana repo with tutorial. Examples can be found here.

Filebeat module

• Test log files exist for the grok patterns
• Generated output for at least 1 log file exists

Metricbeat module

• Example data.json exists and an automated way to generate it exists (go test -data)
• Test environment in Docker exist for integration tests

[mdelapenya]

I'm gonna close this, as it's staled. Reopening if needed