Do not pass the option "device" in rsyslog.conf by default when syslo…

…g server's source address is configured (#17616) (#18514)

### Why I did it

An in-band syslog server will not receive any syslog if it is configured without a VRF specified, which is because `eth0` is always specified as the `device` of a syslog server and the syslog packets will be sent to `eth0` regardless of its destination IP address.

### How I did it

Pass the option "device" in rsyslog.conf only if when syslog server's source address is configured with a non-default VRF

#### How to verify it

Manually test:
1. Configuring a syslog server without VRF specified or with `default` as the VRF: no `device` passed in `rsyslog.conf`
2. Configuring a syslog server with non-default VRF: the configured VRF passed as `device` in `rsyslog.conf`

Co-authored-by: Stephen Sun <5379172+stephenxs@users.noreply.github.com>

files/image_config/rsyslog/rsyslog.conf.j2

@@ -107,19 +107,24 @@ $RepeatedMsgReduction on
 {% set regex = conf.get('filter_regex') -%}
 
 {% set fmodifier = '!' if filter == 'exclude' else '' %}
-{% set device = 'eth0' if vrf == 'default' else vrf -%}
+{% set device = vrf if vrf != '' and vrf != 'default' -%}
 {% set template = 'WelfRemoteFormat' if format == 'welf' else 'SONiCFileFormat' -%}
 
 {# Server extra options -#}
 {% set options = '' -%}
 
 {% if source -%}
     {% set options = options ~ ' Address="' ~ source ~ '"'-%}
+    {% set device = device if device != 'eth0' else '' -%}
+{% endif -%}
+
+{% if device -%}
+    {% set options = options ~ ' Device="' ~ device ~ '"'-%}
 {% endif -%}
 
 {% if filter %}
 :msg, {{ fmodifier }}ereregex, "{{ regex }}"
 {% endif %}
 *.{{ severity }}
-action(type="omfwd" Target="{{ server }}" Port="{{ port }}" Protocol="{{ proto }}" Device="{{ device }}" Template="{{ template }}"{{ options }})
+action(type="omfwd" Target="{{ server }}" Port="{{ port }}" Protocol="{{ proto }}" Template="{{ template }}"{{ options }})
 {% endfor %}