CORD-1868 Add always_pull_xos_images flag

Change-Id: I28c0a7d3d4ece0cfdbbadc8514f435a042d7ad4b
(cherry picked from commit c3afb4e9f54a6ce1f32374ac54548697dcf75a1d)
diff --git a/scenarios/cord/config.yml b/scenarios/cord/config.yml
index bce975e..21ed5b1 100644
--- a/scenarios/cord/config.yml
+++ b/scenarios/cord/config.yml
@@ -119,6 +119,9 @@
 # MAAS isn't installed when prep-headnode runs
 use_apt_cache: False
 
+# Always pull XOS images if start-xos target is re-run
+always_pull_xos_images: True
+
 # images for imagebuilder to build/pull (tagged elsewhere)
 docker_image_whitelist:
   - "xosproject/xos-base"