Move aptly docker images default values to defaults.docker_images

Change-Id: Ifc88a3bef77b7b5998f5ce927f7bad00c589c9ac
Related-bug: PROD-25248
diff --git a/aptly/client/publisher.yml b/aptly/client/publisher.yml
index 050bcea..f30a8ef 100644
--- a/aptly/client/publisher.yml
+++ b/aptly/client/publisher.yml
@@ -6,4 +6,4 @@
       enabled: true
       source:
         engine: docker
-        image: mirantis/aptly-publisher
+        image: ${_param:docker_image_aptly:publisher}