Cherry-pick #20522 to 7.x: Fix ECS fields in Elastic Log Driver, change index prefix

[fearful-symmetry]

Cherry-pick of PR #20522 to 7.x branch. Original message:

What does this PR do?

• Report the correct hostname and properly separated container names in ECS fields
• Change the prefix to logs-docker so documents show up in the Kibana logs UI

Why is it important?

Although the ECS fields themselves are an issue, we also want Log Driver documents to be easily discoverable in Kibana with little additional configuration. The Logs UI will look for logs-* index patterns by default.

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

• Checkout PR locally
• Run mage BuildAndInstall
• Run the plugin using the examples provided in the docs
• Make sure any host.* and container.* fields are correct, and that logs show up in the Kibana log UI
• The end result should look something like this:

{
  "_index": ".ds-logs-docker-8.0.0-2020.08.10-000001",
  "_type": "_doc",
  "_id": "_p2a2nMBsYRBGP3GiK5-",
  "_version": 1,
  "_score": null,
  "_source": {
    "@timestamp": "2020-08-10T23:00:26.046Z",
    "container": {
      "labels": {},
      "id": "6abc8bcd301162fe156d0d560362501a9215c8e9e24e582247ad49dfa9ed1872",
      "name": "awesome_heisenberg",
      "image": {
        "name": "redis",
        "tag": "latest"
      }
    },
    "host": {
      "name": "buzzard.nest"
    },
    "ecs": {
      "version": "1.5.0"
    },
    "agent": {
      "type": "elastic-log-driver",
      "version": "8.0.0",
      "ephemeral_id": "617bcc47-3640-4866-8725-6f22546d53ba",
      "id": "a5308b40-344e-41ab-a985-212075a9762a",
      "name": "buzzard.nest"
    },
    "message": "1:M 10 Aug 2020 23:00:26.045 # WARNING overcommit_memory is set to 0! Background save may fail under low memory condition. To fix this issue add 'vm.overcommit_memory = 1' to /etc/sysctl.conf and then reboot or run the command 'sysctl vm.overcommit_memory=1' for this to take effect."
  },
  "fields": {
    "@timestamp": [
      "2020-08-10T23:00:26.046Z"
    ]
  },
  "sort": [
    1597100426046
  ]
}

[elasticmachine]

Pinging @elastic/integrations (Team:Integrations)

[elasticmachine]

💚 Build Succeeded

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Build stats

• Build Cause: [Pull request #20577 opened]

• Start Time: 2020-08-12T19:06:57.559+0000

• Duration: 28 min 44 sec

Test stats 🧪

Test	Results
Failed	0
Passed	3
Skipped	0
Total	3