Rename allow-secrets to post-review

Since jobs which use secrets in the trusted execution context are
always allowed, the name of this attribute was confusing.  By renaming
it to 'post-review' (and the corresponding job attribute to
'post-review') we indicate what the actual concern is.

Change-Id: I59607621d5b99508b94074133bfc67e64e708a7d
diff --git a/tests/unit/test_scheduler.py b/tests/unit/test_scheduler.py
index 960a922..97d53e0 100755
--- a/tests/unit/test_scheduler.py
+++ b/tests/unit/test_scheduler.py
@@ -2827,7 +2827,7 @@
 
         self.assertHistory([])
         self.assertEqual(A.patchsets[0]['approvals'][0]['value'], "-1")
-        self.assertIn('does not allow jobs with secrets',
+        self.assertIn('does not allow post-review job',
                       A.messages[0])
 
     @simple_layout('layouts/tags.yaml')