image_signing: fix logging multiple args

BUG=None
TEST=ensure_sane_lsb-release.sh errors now include the full string
BRANCH=None

Change-Id: I460ec5e2127a57e7576214fe8fde9e511f940755
Reviewed-on: https://chromium-review.googlesource.com/680038
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: David Riley <davidriley@chromium.org>