Add one-call tracing to log-as-a-file

When a log object is used as a 'dumb' file target, show the origin of the
output.  The motivation here is that 'print' statements are intended to
be an unusual event that should be easily tracked down and eliminated
once their specific use has concluded.
In xcatd, running '-f' means a lot of mysterious output that is hard
to manage as they frequently print out variable contents without
searchable context.  For example, if in xcatd someone randomply prints out
a varibale with a nodename, we might see a stray:
n1
With this change (together with previous changes), the same statement
results in stdout log appearing like:
Jan 19 14:20:54   File "/opt/confluent/lib/python/confluent/plugins/hardwaremanagement/ipmi.py", line 364, in _dict_sensor
    print nodename: n1

confluent_server/confluent/log.py

@@ -1,6 +1,7 @@
 # vim: tabstop=4 shiftwidth=4 softtabstop=4
 
 # Copyright 2014 IBM Corporation
+# Copyright 2015 Lenovo
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -66,6 +67,7 @@ import json
 import os
 import struct
 import time
+import traceback
 
 # on conserving filehandles:
 # upon write, if file not open, open it for append
@@ -235,7 +237,7 @@ class Logger(object):
         :param data: data to log
         """
         if data != '\n':  # 'print' likes to do '\n' by itself, skip that
-            self.log(data)
+            self.log(traceback.format_stack(limit=2)[0][:-1] + ": " + data)
 
     def flush(self):
         pass