Add promote-image-build pipeline

Add a promote pipeline for image builds so that we can use the
image builds that are built in gate.  Without this, we would need
to wait for the next periodic build (or manually request a build)
when build configuration changes.

Change-Id: I707c25a9addae653007f2a2ee2949e7bfc8c37e7

zuul.d/image-pipelines.yaml

@@ -2,6 +2,9 @@
     name: image-build
     manager: independent
     post-review: true
+    description: |
+      This pipeline builds missing images, or builds images on request
+      through the web UI.
     trigger:
       zuul:
         - event: image-build
@@ -10,10 +13,32 @@
         image-built: true
         image-validated: true
 
+- pipeline:
+    name: promote-image-build
+    description: |
+      This pipeline runs jobs that operate after each change is merged
+      in order to promote image builds generated in the gate
+      pipeline.
+    success-message: Build succeeded (promote-image-build pipeline).
+    failure-message: Build failed (promote-image-build pipeline).
+    manager: supercedent
+    precedence: high
+    post-review: True
+    trigger:
+      gerrit:
+        - event: change-merged
+    success:
+      zuul:
+        image-built: true
+        image-validated: true
+      gerrit: {}
+    failure:
+      gerrit: {}
+
 - pipeline:
     name: periodic-image-build
     post-review: true
-    description: Jobs in this queue are triggered on a timer.
+    description: Image build jobs in this pipeline are triggered on a timer.
     manager: independent
     precedence: low
     trigger: