Only run zuul-stream testing on callback changes

Add a file-based exclusion the zuul-stream tests so that we don't bother
running them unless we're changing the callbacks or the callback tests.

Change-Id: I9066fd93f1794519913d9c9fe0ccc201e4f25baf
diff --git a/.zuul.yaml b/.zuul.yaml
index 7f2c589..8718176 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -22,6 +22,9 @@
     post-run: playbooks/zuul-stream/post
     required-projects:
       - openstack/ara
+    files:
+      - "zuul/ansible/callback/.*"
+      - "playbooks/zuul-stream/.*"
 
 - project:
     name: openstack-infra/zuul