Move mcp_docker_registry to defaults

Change-Id: I8645f8df5290112106ef01801f8db86505c327c9
diff --git a/defaults/init.yml b/defaults/init.yml
index 3f2e3f8..9674799 100644
--- a/defaults/init.yml
+++ b/defaults/init.yml
@@ -18,3 +18,5 @@
     linux_system_repo_url: "http://mirror.mirantis.com/${_param:apt_mk_version}/"
     # Openstack globals
     openstack_version: 'queens'
+    # Docker artifact globals
+    mcp_docker_registry: 'docker-prod-local.artifactory.mirantis.com'