container functional test : allow to specify elements-dir

This will allow testing of external elements with this playbook

Change-Id: I6e6aeaa60ca30f10734ed19cb7bd949d621c0636

.zuul.yaml

@@ -35,6 +35,11 @@
 
             The base element to use when building the disk image.
 
+         .. zuul:jobvar:: elements-dir
+            :type: string
+
+            Path to directory with external elements
+
          .. zuul:jobvar:: extra_elements
             :type: list
             :default: []

playbooks/nodepool-functional-container-openstack/templates/nodepool.yaml.j2

@@ -44,6 +44,9 @@ providers:
 diskimages:
   - name: test-image
     rebuild-age: 86400
+{% if 'elements-dir' in nodepool_diskimage %}
+    elements-dir: '{{ nodepool_diskimage.elements-dir }}'
+{% endif %}
     elements:
       - {{ nodepool_diskimage.base_element }}
 {% if 'extra_elements' in nodepool_diskimage %}