Load undercloud images with -d (delete duplicate)

Updates devtest_undercloud.sh so that we use load-image -d
(matches what we do in devtest_overcloud.sh) and is
much more useful when respinning underclouds.

Change-Id: Ie472fab5dd01b73f5701d5058c14fb09c698a6b9
Merge "Load undercloud images with -d (delete duplicate)"