Allow predefined zuul_log_path during swift upload

Some deployments want its own naming schemes like having the project
name if zuul.change is defined. To support that only set the
zuul_log_path if it's undefined yet.

Change-Id: I5f1916bacc12026ce8b4e3f7c7ddeaff6ba7aa06

roles/wip-upload-logs-swift/README.rst

@@ -46,3 +46,8 @@ This uploads logs to an OpenStack Object Store (Swift) container.
 
    If the container is created, should it be created with global read
    ACLs.  If the container already exists, it will not be modified.
+
+.. zuul:rolevar:: zuul_log_path
+   :default: Generated by the role `set-zuul-log-path-fact`
+
+   Prepend this path to the object names when uploading.

roles/wip-upload-logs-swift/tasks/main.yaml

@@ -1,6 +1,7 @@
 - name: Set zuul-log-path fact
   include_role:
     name: set-zuul-log-path-fact
+  when: zuul_log_path is not defined
 
 # Always upload (true), never upload (false) or only on failure ('failure')
 - when: zuul_site_upload_logs | default(true) | bool or