Clean up ConfigParser and reference configs

Some much overdue edits to the Images configuration and additional
features/properties.

- Add `test_image`, `test_disk_format`, `test_container_format` property to images config.
- Add `[images_admin_user]` config section.
- Remove unused config sections/properties.
- Update reference configs.

Change-Id: I63d9ad3d7a243303827448f3b085ba3166317b84
Merge "Clean up ConfigParser and reference configs"