Always update Jenkins node label before deletion.

* modules/openstack_project/files/jenkins_job_builder/config/devstack-gate.yaml:
Run the devstack-vm-inprogress.py script before the
devstack-vm-delete.py script to ensure the Jenkins slave node label is
updated before we delete the node on the cloud provider end.

Change-Id: I1c9bd3ff369b4e595a041ccc687db060a11691e5
Reviewed-on: https://review.openstack.org/29844
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins