Build containers for single consumer job

Make standalone job build containers when there is one single
consumer job in layout. Do not run content provider jobs in
this case.

Change-Id: If3dd4a40d6f85a67134529acd3fafdb598232318

.zuul.yaml

@@ -19,6 +19,7 @@
     description: |
       TripleO standalone Job running Object Storage specific Tempest tests.
     vars:
+      build_container_images: true
       featureset_override:
         run_tempest: false
         tempest_test_whitelist: