[stable/zed] Refactor image tags to be dynamic (#1122)

In order to simplify backporting and branching of new releases,
this patch relies on the Zuul branch when building images so
that we can easily branch out a new release and it will
automatically pick up the images.
Closes: #1121
diff --git a/zuul.d/container-images/nova.yaml b/zuul.d/container-images/nova.yaml
index d16d032..5b415c3 100644
--- a/zuul.d/container-images/nova.yaml
+++ b/zuul.d/container-images/nova.yaml
@@ -46,9 +46,10 @@
           arch:
             - linux/amd64
           build_args:
+            - "RELEASE={{ zuul.branch | replace('stable/', '') }}"
             - PROJECT=nova
           tags:
-            - zed
+            - "{{ zuul.branch | replace('stable/', '') }}"
     files: &container_image_files
       - images/ubuntu/.*
       - images/ubuntu-cloud-archive/.*