Add a periodic image build job

This patch adds a periodic job to build the amphora image using the
released version of diskimage-builder (our other jobs use Git master).
This job will not publish the produced image, it is only to test the
build.

Change-Id: I611d9cd5a519abc6a330ec2c46a2f5a1fdb89313
(cherry picked from commit 1f9915037a)

zuul.d/jobs.yaml

@@ -129,3 +129,17 @@
     required-projects:
       - openstack/octavia-lib
       - openstack/octavia
+
+- job:
+    name: octavia-amphora-image-build
+    parent: base
+    description: |
+      Builds the amphora image using the released diskimage-builder version,
+      not Git master. This job does not publish the image.
+    run: playbooks/image-build/run.yaml
+    required-projects:
+        - openstack/octavia
+        - openstack/octavia-lib
+    vars:
+        amphora_os: ubuntu
+        amphora_os_release: bionic

zuul.d/projects.yaml

@@ -113,3 +113,4 @@
             branches: ^(?!stable/.*).*$
         - publish-openstack-octavia-amphora-image-fedora28:
             branches: ^(?!stable/.*).*$
+        - octavia-amphora-image-build