Create job-output.txt together with JobDir

Otherwise this file will only exist after ansible has started. However
the build url is reported earlier which opens a window where the zuul
status page publishes a live streaming link but the link is not yet
accessible resulting in [1]. Creating the job-output.txt beforehand
should resolve this issue.

[1] Live log result:

  Log not found for build ID 94f4e27243a44e73a19808c328499a28
  --- END OF STREAM ---

Change-Id: I9c85a7a1aa925d6cf8080ef07f1855d28fa3c71a
1 file changed