dib-functional : allow extra elements to be passed

Add a variable to make it easier to add extra elements for testing
purposes.

Change-Id: Ib246228214beff0aaf2a9cef3df4459657b1e93f

.zuul.yaml

@@ -35,6 +35,13 @@
 
             The base element to use when building the disk image.
 
+         .. zuul:jobvar:: extra_elements
+            :type: list
+            :default: []
+
+            List of extra DIB elements to be added to image
+            configuration.
+
          .. zuul:jobvar:: release
 
             The release number or name to use with the base element.

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

@@ -46,6 +46,11 @@ diskimages:
     rebuild-age: 86400
     elements:
       - {{ nodepool_diskimage.base_element }}
+{% if 'extra_elements' in nodepool_diskimage %}
+{%   for item in nodepool_diskimage.extra_elements %}
+      - {{ item }}
+{%   endfor %}
+{% endif %}
       - vm
       - simple-init
       - growroot