Test and document behavior of merging agent plugin.config stanzas

[schmichael]

Nomad version

v0.12.8

Issue

Given 2 agent config files:

00-docker.hcl

plugin "docker" {
  config {
    volumes {
      enabled = true
    }
  }
}

01-docker.hcl

plugin "docker" {
  config {
    endpoint = "unix:///var/run/docker.sock"
   }
}

Is the merged config additive:

plugin["docker"].config.volumes.enabled = true
plugin["docker"].config.endpoint = "..."

Or Last Write Wins:

plugin["docker"].config.endpoint = "..."

The former (merged) is expected, but we may be better off updating documentation than breaking backward compatibility.

[tgross]

Configuration file file1.hcl:

plugin "docker" {
  config {
    allow_privileged = true
  }
}

Configuration file file2.hcl:

plugin "docker" {
  config {
    volumes {
      enabled = true
    }
  }
}

Results, which demonstrate that we don't merge:

$ nomad node status -self -verbose | grep docker
docker       true      true     Healthy  2020-12-18T16:19:48Z
driver.docker                 = 1
driver.docker.bridge_ip       = 172.17.0.1
driver.docker.os_type         = linux
driver.docker.runtimes        = runc
driver.docker.version         = 19.03.8
driver.docker.volumes.enabled = true

Working up docs for this now. It looks like this behavior is inconsistent... it applies to plugins but not, for example advertise.

[github-actions]

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