Refactor agent.Config to maintain user config vs normalized settings

[schmichael]

Nomad currently alters agent.Config in various places to normalize/autodetect various settings.

• agent/Config.normalizeAddrs changes advertise addresses based on a number of factors
• agent/Command.readConfig sets a few defaults

Ideally the original config would be maintained as-is and normalized/derived settings would either be in internal configs like nomad.Config or at least on unexported fields on the agent config.

[github-actions]

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