Reapply "Isolated task support in Endpoints API: client side (3/3)" and fixes"

client/example/payload/hello_world.py

Index: client/example/payload/hello_world.py
diff --git a/client/example/payload/hello_world.py b/client/example/payload/hello_world.py
index 86b15b382bed242860e4b13196f18a2784579314..d8a49d7ed00aaea2cf770a70710073f6a4fde791 100755
--- a/client/example/payload/hello_world.py
+++ b/client/example/payload/hello_world.py
@@ -3,13 +3,21 @@
 # Use of this source code is governed under the Apache License, Version 2.0 that
 # can be found in the LICENSE file.
 
-"""This script is meant to be run on a Swarm slave."""
+"""This script is meant to be run on a Swarming slave."""
 
+import os
 import sys
 
 
 def main():
   print('Hello world: ' + sys.argv[1])
+  if len(sys.argv) == 3:
+    # Write a file in ${ISOLATED_OUTDIR}.
+    with open(os.path.join(sys.argv[2], 'happiness.txt'), 'wb') as f:
+      f.write(
+          'is where you look %d/%d' % (
+            int(os.environ['GTEST_SHARD_INDEX']),
+            int(os.environ['GTEST_TOTAL_SHARDS'])))
   return 0