fix iscsi-boot element exiting build even if dracut-regenerate used

in CentOS build case building an image with "iscsi-boot" and "dracut-regenerate" will exit building because of statement "[ "$found" = 0 ]"

Change-Id: I1a6d60e9ec5f5cb508866c8376465c3e73551a30

diskimage_builder/elements/iscsi-boot/extra-data.d/50-check-dracut-regenerate

@@ -8,10 +8,8 @@ set -o pipefail
 
 if [[ $DISTRO_NAME = "centos7" ]]; then
     eval declare -A image_elements=($(get_image_element_array))
-    found=0
-    for i in "${!image_elements[@]}"; do
-        [ "$i" = "dracut-regenerate" ] && found=1
-    done
-    [ "$found" = 0 ] && echo "The dracut-regenerate element is required!" && exit 1
-
+    if [[ ! "${image_elements[@]}" =~ "dracut-regenerate" ]]; then
+        echo "The dracut-regenerate element is required!"
+        exit 1
+    fi
 fi