Cherry-pick #20908 to 7.x: [Filebeat] backwards compatibility for set processor

[leehinman]

Cherry-pick of PR #20908 to 7.x branch. Original message:

What does this PR do?

When loading a pipeline this change checks the elasticsearch version
and if the version is less than 7.9.0 it will replace the
"ignore_empty_value" option with an equivalent if statement on the set
processor.

Why is it important?

This allows filebeat > 7.9.0 to be used with older versions of
elasticsearch. Without it the pipelines fail to load because the
option isn't supported.

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.

How to test this PR locally

run filebeat modules enable zeek && filebeat -e setup

Logs

Error

2020-09-01T12:42:48.781-0500	ERROR	fileset/setup.go:81	Error loading pipeline: 1 error: Error loading pipeline for fileset zeek/x509: couldn't load pipeline: couldn't load json. Error: 400 Bad Request: {"error":{"root_cause [{"type":"parse_exception","reason":"processor [set] doesn't support one or more provided configuration parameters [ignore_empty_value]",
...

With fix:

2020-09-01T14:04:56.470-0500	DEBUG	[modules] fileset/pipelines.go:277     	in pipeline filebeat-8.0.0-zeek-x509-pipeline replacing unsupported 'ignore_empty_value' with if ctx?.zeek?.x509?.certificate?.subject?.state != null in set processor

[elasticmachine]

Pinging @elastic/siem (Team:SIEM)

[elasticmachine]

💚 Build Succeeded

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Build stats

• Build Cause: [Pull request #20969 opened]

• Start Time: 2020-09-03T18:09:02.968+0000

• Duration: 59 min 40 sec

Test stats 🧪

Test	Results
Failed	0
Passed	5579
Skipped	833
Total	6412