Bring back tools.run(), but only conditionally on gflags.

Add a warning that tools.run() is deprecated and will be removed in a future
version.

Reviewed in https://codereview.appspot.com/9772046/.
diff --git a/apiclient/sample_tools.py b/apiclient/sample_tools.py
index ff17b2b..ba6d754 100644
--- a/apiclient/sample_tools.py
+++ b/apiclient/sample_tools.py
@@ -85,7 +85,7 @@
   storage = file.Storage(name + '.dat')
   credentials = storage.get()
   if credentials is None or credentials.invalid:
-    credentials = tools.run(flow, storage, flags)
+    credentials = tools.run_flow(flow, storage, flags)
   http = credentials.authorize(http = httplib2.Http())
 
   # Construct a service object via the discovery service.