archive-release: for minnow, drop ${RELEASE_IMAGE}.iso

We only care about the hard disk image (hddimg), not the iso, so don't bother
shipping it. This frees up some space in our release artifacts.

JIRA: SB-2976

Signed-off-by: Christopher Larson <kergoth@gmail.com>
Merge pull request #259 from kergoth/sb-2976

archive-release: for minnow, drop ${RELEASE_IMAGE}.iso