Add default for build diskimage image name

Use `zuul.image_build_name` as default for the
`build_diskimage_image_name` variable, so that is enough for jobs to
define `job.image-build-name`.

If `zuul.image_build_name` is not set we require
`build_diskimage_image_name`to be defined.

Change-Id: I837b5d1571dce38801d6a0433e960734f573fedc

roles/build-diskimage/README.rst

@@ -27,6 +27,7 @@ Example:
    {{ ensure_dib_command }}. as being set by the ensure-dib role.
 
 .. zuul:rolevar:: build_diskimage_image_name
+   :default: "{{ zuul.image_build_name }}"
 
    Name of the image to build.
 

roles/build-diskimage/defaults/main.yaml

@@ -1,4 +1,7 @@
 build_diskimage_command: "{{ ensure_dib_command }}"
+# If used within a non-image-build context this variable needs to be explicitly
+# defined. The role uses a 'mandatory' filter to ensure it is set
+build_diskimage_image_name: "{{ zuul.image_build_name }}"
 
 build_diskimage_formats:
   - qcow2

zuul.d/dib-jobs.yaml

@@ -11,6 +11,7 @@
       or prepare it to be uploaded to log storage.
 
       .. zuul:jobvar:: build_diskimage_image_name
+         :default: "{{ zuul.image_build_name }}"
 
          Name of the image to build.