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/fixtures/layouts/untrusted-secrets.yaml b/tests/fixtures/layouts/untrusted-secrets.yaml
index cfa03e0..b90d3d7 100644
--- a/tests/fixtures/layouts/untrusted-secrets.yaml
+++ b/tests/fixtures/layouts/untrusted-secrets.yaml
@@ -17,7 +17,7 @@
 
 - job:
     name: project1-test
-    untrusted-secrets: true
+    post-review: true
 
 - project:
     name: org/project1