Gate heterogeneous payloads on group metadata

In order for the pipeline to attempt a heterogeneous release payload,
group metadata must indicate the group's/assembly's support of the
payload format.

multi_arch:
  enabled: true

doozerlib/cli/release_gen_payload.py

@@ -540,6 +540,10 @@ def update_single_arch_istags(apiobj: oc.APIObject):
         if not apply_multi_arch:
             break
 
+        if not runtime.group_config.multi_arch.enabled:
+            logger.info("--apply-multi-arch is enabled but the group config / assembly does not have group.multi_arch.enabled==true")
+            break
+
         imagestream_name, imagestream_namespace = payload_imagestream_name_and_namespace(
             base_imagestream_name,
             base_istream_namespace,