Reduce tools/image_list.sh down to 4 images

Now that mysql.qcow2 has been removed, we only have 4 images to worry
about. This fixes cache-devstack element for openstack-infra.

Change-Id: Ia06f0e0679e253a1a6614f7c38abf1f5cd13991b
Signed-off-by: Paul Belanger <pabelanger@redhat.com>
diff --git a/tools/image_list.sh b/tools/image_list.sh
index 27b3d46..29b93ed 100755
--- a/tools/image_list.sh
+++ b/tools/image_list.sh
@@ -36,7 +36,7 @@
 
 # Sanity check - ensure we have a minimum number of images
 num=$(echo $ALL_IMAGES | tr ',' '\n' | sort | uniq | wc -l)
-if [[ "$num" -lt 5 ]]; then
+if [[ "$num" -lt 4 ]]; then
     echo "ERROR: We only found $num images in $ALL_IMAGES, which can't be right."
     exit 1
 fi