Update documentation for using diskimage-builder

Recent change in nodepool to build images using
diskimage-builder involved adding new diskimages section
in the nodepool.yaml file, and added new commands like
dib-image-list, dib-image-delete, image-build and image-upload.

Change-Id: If36f3f6e39e382cb4c6398d3a063c979888a8642
Merge "Update documentation for using diskimage-builder"