Remove systemd.source prefix from logs tag

To simplify filtering in Kibana change
systemd.source prefix to record field "source".

Change-Id: I7729ae6721a1050a938370a588d35313f91f971a
Related-bug: PROD-21827 (PROD:21827)

linux/meta/fluentd.yml

             record:
               - name: severity_label
                 value: '${ {"TRACE"=>8,"DEBUG"=>7,"INFO"=>6,"NOTICE"=>5,"WARNING"=>4,"ERROR"=>3,"CRITICAL"=>2,"ALERT"=>1,"EMERGENCY"=>0}.key(record["Severity"].to_i) }'
+              - name: source
+                value: systemd
         match:
           rewrite_tag:
             tag: systemd.source
             rule:
               - name: ident
                 regexp: '^(.*)$'
-                result: __TAG__.$1
+                result: $1.systemd
           push_to_default:
-            tag: 'systemd.source.*'
+            tag: '*.systemd'
             type: copy
             store:
               - type: relabel