Ensure logs are uploaded on failure

As per http://ci.openstack.org/jenkins-job-builder/publishers.html#publishers.postbuildscript*
we need to set onfailure to true to upload logs in all cases.

* The documentation is actually currently wrong. I've posted a fix
here: https://review.openstack.org/138952

Change-Id: Iaf5413a88ed47792fbb44d7937191ab001e15200