Cherry-pick #18467 to 7.x: [Journalbeat] Add ID config option input

[andrewkroh]

Cherry-pick of PR #18467 to 7.x branch. Original message:

What does this PR do?

Add an id configuration option to the Journalbeat input to make it possible
to start multiple inputs on the same journal.

I also fixed a test that was modifying a file under source control by copying it
into the build directory before applying changes.

Why is it important?

As an administrator this provides more flexibility in how you can deploy
Journalbeat. It gives you the configuration grainularity you need to be able
to have an input for each service or log type. You can apply different
include_matches to each input. And you can easily add new log types at any
point and make Journalbeat read all matching logs from the "head" without
affecting existing input cursor positions.

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

Relates: #10985

Use cases

The configuration I push out to Journalbeat depends on what particular services are deployed on that node. This allows me to setup one input for each service.

journalbeat.inputs:
- id: vault.service
  input_matches: [_SYSTEMD_UNIT=vault.service]
  processors:
    - disect: ...

- id: coredns.service
  input_matches: [_SYSTEMD_UNIT=coredns.service]
  processors:
    - disect: ...

- id: iptables
  input_matches:
    - _TRANSPORT=kernel
    - SYSLOG_IDENTIFIER=kernel
  processors:
    - drop_event:
        when.not.regexp.message: '^iptables'
    - dissect: ...

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Pinging @elastic/integrations-services (Team:Services)

[elasticmachine]

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Build stats

• Build Cause: [Pull request Cherry-pick #18467 to 7.x: [Journalbeat] Add ID config option input #18590 opened]

• Start Time: 2020-05-15T15:35:56.925+0000

• Duration: 28 min 52 sec (1671806)