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

client/example/payload/hello_world.isolate

 # Copyright 2012 The Swarming Authors. All rights reserved.
 # Use of this source code is governed under the Apache License, Version 2.0 that
 # can be found in the LICENSE file.
 {
   'variables': {
     'files': [
       'hello_world.py',
     ],
   },
   #
   'conditions': [
-    ['OS=="Linux"', {
+    ['OS=="Ubuntu"', {
       'variables': {
         'command': [
           'python',
           'hello_world.py',
-          'Linux',
+          'Ubuntu',
         ],
       },
     }],
     ['OS=="Mac"', {
       'variables': {
         'command': [
           'python',
           'hello_world.py',
           'OSX',
         ],
@@ skipping 12 matching lines @@
       'variables': {
         'command': [
           'python',
           'hello_world.py',
           'Windows',
         ],
       },
     }],
   ],
 }