[rsyslog]: Use timegenerated instead of timestamp (#944)

* [rsyslog]: Use timegenerated instead of timestamp

This is useful when rsyslog is used to put markers generated on other machines.
This way all messages will have a timestamp from a single system.

* [rsyslog] Use subseconds from local machine

files/image_config/rsyslog/rsyslog.conf.j2

@@ -37,7 +37,7 @@ $UDPServerRun 514
 #$ActionFileDefaultTemplate RSYSLOG_TraditionalFileFormat
 
 # Define a custom template
-$template SONiCFileFormat,"%TIMESTAMP%.%timestamp:::date-subseconds% %HOSTNAME% %syslogseverity-text:::uppercase% %syslogtag%%msg:::sp-if-no-1st-sp%%msg:::drop-last-lf%\n"
+$template SONiCFileFormat,"%timegenerated%.%timegenerated:::date-subseconds% %HOSTNAME% %syslogseverity-text:::uppercase% %syslogtag%%msg:::sp-if-no-1st-sp%%msg:::drop-last-lf%\n"
 $ActionFileDefaultTemplate SONiCFileFormat
 
 #Set remote syslog server