[Filebeat] Document netflow internal_networks and set default

[andrewkroh]

What does this PR do?

Documentation for the internal_networks option of the Netflow input and module was missing.
Also the module's manifest did not declare the option so if it was not set in the module config
it would cause an error.

I did not see where a default was set for the netflow input's internal_networks option so I set that
to private to keep the old behavior before this was configurable.

Fixes #24094

Why is it important?

Existing users of the netflow module are broken after upgrade unless they change their config.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Related issues

• Fixes [filebeat][netflow] filebeat does't start when internal_networks variable isn't defined in config file #24094

[elasticmachine]

Pinging @elastic/security-external-integrations (Team:Security-External Integrations)

[andrewstucki]

@andrewkroh wow, not sure how we didn't catch this previously, but I checked over the previous changes again and we also look like we have an issue with the configuration. In filebeat/module/netflow/log/config/netflow.yml:

{{if .internal_networks}}
internal_hosts:
{{range .internal_networks}}
- '{{ . }}'
{{end}}
{{end}}

notice the internal_hosts v. internal_networks. When the input has:

 InternalNetworks []string `config:"internal_networks"`

Any chance you could add a fix for that here too? Otherwise it's not configurable at all 😞

[andrewstucki]

LGTM

[elasticmachine]

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• Build Cause: Started by user Andrew Stucki

• Start Time: 2021-02-19T22:19:43.973+0000

• Duration: 48 min 34 sec

• Commit: d10a383

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Total	15271

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Total	15271