Add systemd's _HOSTNAME field as hostname to log events

See #16

Author: jvassev

config-reloader/templates/systemd.conf

@@ -10,7 +10,7 @@
   tag systemd.unit
   read_from_head false
   <entry>
-    field_map {"_SYSTEMD_UNIT": "unit", "MESSAGE": "log", "_PID": ["pid"], "_PRIORITY": "priority", "_COMM": "cmd"}
+    field_map {"_SYSTEMD_UNIT": "unit", "MESSAGE": "log", "_PID": ["pid"], "_PRIORITY": "priority", "_COMM": "cmd", "_HOSTNAME": "hostname" }
     field_map_strict true
     fields_lowercase true
   </entry>