Add DIB_DEBOOTSTRAP_EXTRA_ARGS environment variable

This allows extra arguments to be passed to the debootstrap
command when creating an image using the debian element.

Change-Id: I87eb856365ff6a17f051e2e56587235648328d57
Co-Authored-By: Ghe Rivero <ghe.rivero@gmail.com>
Merge "Add DIB_DEBOOTSTRAP_EXTRA_ARGS environment variable"