Make all devstack-vm jobs single-use

Use the new gearman-plugin single-use feature with all 'devstack-vm'
jobs.  This is compatible with the current devstack-gate scripts,
though once this is in place, they can be updated so that they no
longer perform relabel operations.

This is also compatible with future devstack-gate nodepool operations.

Change-Id: I6fcadb47ba5eeead3d11926019b3c041305c95f7

modules/openstack_project/files/zuul/layout.yaml

@@ -85,6 +85,8 @@ pipelines:
 jobs:
   - name: ^.*$
     parameter-function: set_log_url
+  - name: ^.*devstack-vm.*$
+    parameter-function: single_use_node
   - name: gate-tempest-devstack-vm-cinder
     branch: stable/folsom
   - name: gate-tempest-devstack-vm-smoke

modules/openstack_project/files/zuul/openstack_functions.py

@@ -1,3 +1,17 @@
+# Copyright 2013 OpenStack Foundation
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you may
+# not use this file except in compliance with the License. You may obtain
+# a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+# License for the specific language governing permissions and limitations
+# under the License.
+
 def set_log_url(item, job, params):
     if hasattr(item.change, 'refspec'):
         path = "%s/%s/%s/%s" % (
@@ -12,3 +26,8 @@ def set_log_url(item, job, params):
     params['BASE_LOG_PATH'] = path
     params['LOG_PATH'] = path + '/%s/%s' % (job.name,
                                             params['ZUUL_UUID'][:7])
+
+
+def single_use_node(item, job, params):
+    set_log_url(item, job, params)
+    params['OFFLINE_NODE_WHEN_COMPLETE'] = '1'