Add a role to emit an informative header for logs

It contains the URL of the change being tested as well as the location
the logs are going to be found in.

Change-Id: I28bcd1c004bc399d3404e3c6a1f44fdfe98cf97d

roles/emit-job-header/tasks/main.yaml

@@ -0,0 +1,7 @@
+- debug:
+    msg: "Job {{ zuul.job }} triggered from {{ zuul.change_url }}"
+  when: zuul.change_url is defined
+
+- debug:
+    msg: "At completion, logs for this job will be availble at {{ log_path }}"
+  when: log_path is defined