Fix oversight in container format handling

https://review.openstack.org/5934 changed the container and disk format handling,
but I forgot to reset the variables at the top of the loop (DOH!)

Change-Id: I2782c06e7b4da1b69ecb72f847a5593bda522b7d
diff --git a/stack.sh b/stack.sh
index 2766744..1a9b0c6 100755
--- a/stack.sh
+++ b/stack.sh
@@ -1659,6 +1659,8 @@
 
         KERNEL=""
         RAMDISK=""
+        DISK_FORMAT=""
+        CONTAINER_FORMAT=""
         case "$IMAGE_FNAME" in
             *.tar.gz|*.tgz)
                 # Extract ami and aki files