Use Ansible's features for "new sources" preparation

Change-Id: I33f47c65f2bb609798fee8df7f700b3dad3e46d3
diff --git a/zuul.yaml b/zuul.yaml
index cf2f5a6..9524560 100644
--- a/zuul.yaml
+++ b/zuul.yaml
@@ -54,6 +54,7 @@
     failure-url: coverage-diff.html
     vars:
       coverage_xml_dir: "{{ ansible_user_dir }}/build"
+      new_source_prefix: "{{ ansible_user_dir }}/new"
 
 - job:
     name: f29-gcc-asan