Refactor all gRPC proxy code into a single class.

appengine/swarming/swarming_bot/bot_code/remote_client.py

Index: appengine/swarming/swarming_bot/bot_code/remote_client.py
diff --git a/appengine/swarming/swarming_bot/bot_code/remote_client.py b/appengine/swarming/swarming_bot/bot_code/remote_client.py
index 579e537a69583adca224ada7aff4d44ec62047d5..610b86f538f18b84cfb9ab17fdb8e5f274d15c73 100644
--- a/appengine/swarming/swarming_bot/bot_code/remote_client.py
+++ b/appengine/swarming/swarming_bot/bot_code/remote_client.py
@@ -4,6 +4,7 @@
 
 import base64
 import logging
+import os
 import threading
 import time
 import traceback
@@ -42,6 +43,7 @@ NET_CONNECTION_TIMEOUT_SEC = 3*60
 
 
 def createRemoteClient(server, auth, grpc_proxy):
+  grpc_proxy = os.environ.get('SWARMING_GRPC_PROXY', grpc_proxy)

[M-A Ruel, 2017/08/02 21:05:11]

We'd need to document these under doc/

[aludwin, 2017/08/04 01:44:26]

Done.

   if grpc_proxy:
     import remote_client_grpc
     return remote_client_grpc.RemoteClientGrpc(grpc_proxy)