Allow dib_elements key to be a nested list

It's highly likely that folks may want to use YAML anchors to
build up list of DIB elements.  To aid in that, allow the value
to be a list of lists and automatically flatton it.

Change-Id: I55b9cb16951b51da32f99ca5858b75217951b279

roles/build-diskimage/README.rst

@@ -42,6 +42,9 @@ Example:
 
    List of elements that should be used when creating the disk image.
 
+   For ease of use with YAML anchors, this may be provided as a list
+   of lists which will be flattened automatically.
+
 .. zuul:rolevar:: build_diskimage_environment
    :type: dict
 

roles/build-diskimage/tasks/main.yaml

@@ -9,7 +9,7 @@
       -o "{{ build_diskimage_image_root }}/{{ build_diskimage_image_name | mandatory }}"
       -t "{{ build_diskimage_formats | join(',') }}"
       --logfile "{{ build_diskimage_logs_dir }}/diskimage-{{ build_diskimage_image_name }}.log"
-      {{ build_diskimage_elements | mandatory | join(' ') }}
+      {{ build_diskimage_elements | mandatory | flatten | join(' ') }}
   args:
     chdir: "{{ build_diskimage_image_root }}"
   environment: "{{ build_diskimage_environment }}"

zuul.d/dib-jobs.yaml

@@ -26,6 +26,9 @@
 
          List of elements that should be used when creating the disk image.
 
+         For ease of use with YAML anchors, this may be provided as a list
+         of lists which will be flattened automatically.
+
       .. zuul:jobvar:: build_diskimage_environment
          :type: dict