Avoid generating incomplete configurations in autodiscover

[jsoriano]

What does this PR do?

Handle errors when configuration unpacking fails. In principle this can
only happen when some variable is missing, because configuration has
been previously parsed as YAML. Errors on unpacking were previously
ignored.

When a variable is missing, this is clearly logged at the debug level.

This changes the behaviour, previously an incomplete configuration was
generated on this case.

Why is it important?

To detect problems with incomplete configuration earlier and improve feedback in logs
when a variable is missing in event.

Since #18979 this error is more frequent, as a legit configuration for containers produces
errors with pod events that don't contain container fields.

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

• Related to Fix metrics hints builder to avoid wrong container metadata usage when port is not exposed #18979.
• Fixes Improve logging when autodiscover configs fail #20568.

Logs

For example the following configuration includes an incorrect field container.id.

filebeat.autodiscover:
  providers:
    - type: docker
      hints.enabled: true
      hints.default_config:
        type: container
        paths:
          - /var/lib/docker/containers/${container.id}/*.log

Before this change, an incomplete configuration is generated, and errors like these ones are logged:

2020-09-01T16:52:50.094+0200	ERROR	[autodiscover]	autodiscover/autodiscover.go:209	Auto discover config check failed for config '{
  "docker-json": {
    "cri_flags": true,
    "format": "auto",
    "partial": true,
    "stream": "all"
  },
  "symlinks": true,
  "type": "container"
}', won't start runner: each input must have at least one path defined

After this change, the cause of the problem is more clear, and it is only logged at the debug level:

2020-09-02T11:57:08.225+0200	DEBUG	[autodiscover]	template/config.go:156	Configuration template cannot be resolved: field 'container.id' not available in event or environment accessing 'paths' (source:'/home/jaime/tmp/filebeat-autodiscover-docker-hints.yml')

[elasticmachine]

Pinging @elastic/integrations-platforms (Team:Platforms)

[elasticmachine]

💚 Build Succeeded

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Build stats

• Build Cause: [Pull request #20898 updated]

• Start Time: 2020-09-02T09:54:23.224+0000

• Duration: 66 min 4 sec

Test stats 🧪

Test	Results
Failed	0
Passed	19661
Skipped	1830
Total	21491

[ChrsMark]

Looks good to me! Thanks for fixing this! I just left a nit question.