syslog: T6719: fix the behavior of "syslog global preserve-fqdn" (backport #4079)

[mergify]

Change Summary

Add config option to set local host name for syslog

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

• https://vyos.dev/T6719

Related PR(s)

Component(s) name

system

How to test

set system domain-name example.local
set system syslog global preserve-fqdn

Adds the rsyslog config entry

$LocalHostName vyos.example.local

Smoketest result

vyos@vyos:~$ /usr/libexec/vyos/tests/smoke/cli/test_system_syslog.py
test_syslog_basic (__main__.TestRSYSLOGService.test_syslog_basic) ... ok
test_syslog_global (__main__.TestRSYSLOGService.test_syslog_global) ... ok

----------------------------------------------------------------------
Ran 2 tests in 21.041s

OK

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

---

This is an automatic backport of pull request #4079 done by [Mergify](https://mergify.com).

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