Add post-review pipeline

In order to start using jobs with secrets before gating it required to
have a special post review pipeline. This pipeline can be triggered by
setting Allow-Post-Review gerrit flag.

To address potential races for Verified flag this pipeline does not set
it.

For the proper setup jobs of the post-review pipeline should also appear
in gate pipeline.

Change-Id: Ib633a621fbf004f2a34b8826014e3625c093c21f

zuul.d/pipelines.yaml

@@ -56,6 +56,31 @@
         check: 'failure'
         comment: false
 
+- pipeline:
+    name: post-review
+    description: |
+      Patches received initial review by project trusted members enter this
+      pipeline to execute additional jobs. This pipeline can be used to access
+      secrets before gating.
+    success-message: Build succeeded (post-review pipeline).
+    failure-message: |
+      Build failed (post-review pipeline). For information on how to proceed, see
+      https://docs.opendev.org/opendev/infra-manual/latest/developers.html#automated-testing
+    manager: independent
+    post-review: True
+    precedence: low
+    require:
+      gerrit:
+        open: True
+        current-patchset: True
+        approval:
+          - Allow-Post-Review: [1]
+    trigger:
+      gerrit:
+        - event: comment-added
+          require-approval:
+            - Allow-Post-Review: [1]
+
 - pipeline:
     name: gate
     description: |