Add image build pipelines

This adds pipelines that we can attach image build jobs to.

Change-Id: Ic4af83c5af42e915b58471a000131297f367ab7e

zuul.d/image-pipelines.yaml

@@ -0,0 +1,25 @@
+- pipeline:
+    name: image-build
+    manager: independent
+    post-review: true
+    trigger:
+      zuul:
+        - event: image-build
+    success:
+      zuul:
+        image-built: true
+        image-validated: true
+
+- pipeline:
+    name: periodic-image-build
+    post-review: true
+    description: Jobs in this queue are triggered on a timer.
+    manager: independent
+    precedence: low
+    trigger:
+      timer:
+        - time: '0 2 * * *'
+    success:
+      zuul:
+        image-built: true
+        image-validated: true