job now supports a custom workspace

In Jenkins, the job advanced options has a tick box to enable a
workspace space different from the default provided internally by
Jenkins.  The XML element is <customWorkspace />.

Change-Id: Ia432d98523a7cb617c1236acb52821a82b67c8e7
Reviewed-on: https://review.openstack.org/24427
Reviewed-by: Arnaud Fabre <fabre.arnaud@gmail.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: Khai Do <zaro0508@gmail.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins