Refactor image tags to be dynamic (#1120)

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.
diff --git a/zuul.d/container-images/ovn.yaml b/zuul.d/container-images/ovn.yaml
index 67eaaec..5a3e180 100644
--- a/zuul.d/container-images/ovn.yaml
+++ b/zuul.d/container-images/ovn.yaml
@@ -38,18 +38,20 @@
           arch:
             - linux/amd64
           build_args:
+            - "RELEASE={{ zuul.branch | replace('stable/', '') }}"
             - OVN_COMPONENT=central
           tags:
-            - main
+            - "{{ zuul.branch | replace('stable/', '') }}"
         - context: images/ovn
           registry: registry.atmosphere.dev
           repository: registry.atmosphere.dev/library/ovn-host
           arch:
             - linux/amd64
           build_args:
+            - "RELEASE={{ zuul.branch | replace('stable/', '') }}"
             - OVN_COMPONENT=host
           tags:
-            - main
+            - "{{ zuul.branch | replace('stable/', '') }}"
     files: &container_image_files
       - images/openvswitch/.*
       - images/ovn/.*