AWS: Add sha to download-or-bust

download-or-bust was changed on the GCE side to verify a hash if passed,
so we need to pass a hash in the AWS scripts that reuse this function.

cluster/gce/configure-vm.sh

@@ -240,7 +240,7 @@ install-salt() {
 
   for deb in "${DEBS[@]}"; do
     if [ ! -e "${deb}" ]; then
-      download-or-bust "${URL_BASE}/${deb}"
+      download-or-bust "" "${URL_BASE}/${deb}"
     fi
   done