ara-report: add option for artifact prefix

After https://review.opendev.org/#/c/742971 the report is generated on
the host.  For system-config, this means the report ends up copied to
"bridge.openstack.org/ara-report" in the logs directory; but the
artifact points to the non-existant top-directory.

Add a variable to allow a top-level dir to be set.

Needed-By: https://review.opendev.org/747101
Change-Id: I61a7d7328b5a45fc8dee09909f9411c397dd31b6

roles/ara-report/README.rst

@@ -60,3 +60,10 @@ If ARA is enabled, generates a report or saves a copy of the ARA database.
    :default: ``ara``
 
    Path to ara executable.
+
+.. zuul:rolevar:: ara_report_artifact_prefix
+   :default: None
+
+   Prefix to add to the artifact URL reported to Zuul.  If you copy
+   results into a subdirectory, add this here.  Note this should have
+   a trailing ``/``.

roles/ara-report/defaults/main.yaml

@@ -25,3 +25,7 @@ ara_report_path: '{{ zuul.executor.log_root }}/ara'
 ara_compress_html: true
 
 ara_report_executable: ara
+
+# If you copy the ara results into a subdirectory, use this to prefix
+# the location the artifact points to.
+ara_report_artifact_prefix: ''

roles/ara-report/tasks/main.yaml

@@ -63,7 +63,7 @@
           zuul:
             artifacts:
               - name: ARA report
-                url: "{{ ara_report_path | regex_replace('\\/$', '') | basename }}"
+                url: "{{ ara_report_artifact_prefix }}{{ ara_report_path | regex_replace('\\/$', '') | basename }}"
   rescue:
     - name: HTML generation rescue
       debug: