Backport of fix: Make agent syslog log level inherit from Nomad agent log into release/1.4.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #15625 to be assessed for backporting due to the inclusion of the label backport/1.4.x.

The below text is copied from the body of the original PR.

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Fix issue : #15087

Hi Nomad Core Dev team,

Below is my attempt to fix the issue mentioned above. Imo, we can either make an extra argument for syslog log level or make it inherit from Nomad Agent existing one. This PR is using the latter approach because there is not need to use the former for a rarely used feature.

Would appreciate getting your feedback on this PR, especially on a few issues:

• On the mapping between gsyslog and Nomad Agent log, I'm not entirely sure about how OFF and TRACE in Nomad log_level maps to gsyslog log level
• I attempted to write one similar to TestCommand_Args() but the log is not captured by either ui.ErrorWriter or ui.OutputWriter. Please let me know if a unit test is needed or there are better ways to structure the unit test

[github-actions]

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