agent: server ID in banner should be persistent

[tgross]

During agent startup on a server, we log the server node ID in the banner. But instead of using the node ID we got during the initial setup (or from the persistent ID on disk), we regenerate the ID. In fact, we regenerate the entire configuration for the purpose of displaying this one log line and then throw it out!

Fix this by using the node ID already available on the server.

Testing & Reproduction steps

$ nomad server members -json | jq '.[0].Tags.id'
"3d90ad98-70a1-7366-9e30-b980c4bea01c"

Logs:

==> Loaded configuration from /home/tim/ws/nomad/etc/local/server.hcl
==> Starting Nomad agent...
==> Nomad agent configuration:

       Advertise Addrs: HTTP: 192.168.1.194:4646; RPC: 192.168.1.194:4647; Serf: 192.168.1.194:4648
            Bind Addrs: HTTP: [0.0.0.0:4646]; RPC: 0.0.0.0:4647; Serf: 0.0.0.0:4648
                Client: false
             Log Level: debug
               Node Id: 3d90ad98-70a1-7366-9e30-b980c4bea01c
                Region: philly (DC: dc1)
                Server: true
               Version: 2.0.5-dev

Logs after restart:

==> Nomad agent configuration:

       Advertise Addrs: HTTP: 192.168.1.194:4646; RPC: 192.168.1.194:4647; Serf: 192.168.1.194:4648
            Bind Addrs: HTTP: [0.0.0.0:4646]; RPC: 0.0.0.0:4647; Serf: 0.0.0.0:4648
                Client: false
             Log Level: debug
               Node Id: 3d90ad98-70a1-7366-9e30-b980c4bea01c
                Region: philly (DC: dc1)
                Server: true
               Version: 2.0.5-dev

Contributor Checklist

• Changelog Entry If this PR changes user-facing behavior, please generate and add a
  changelog entry using the make cl command.
• Testing see above
• Documentation n/a
• LLM Usage n/a

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  within the public repository.

• If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

[jrasell]

LGTM!