Add success message to instack-build-images

When image builds complete successfully, there's no obvious way to
know other than checking $?.  This adds an echo after the build
calls, which thanks to set -e should only be run if everything
completes successfully.

Change-Id: I4fac0b05b7340b12447f3a23747a2a347bc0d33f

scripts/instack-build-images

@@ -298,6 +298,8 @@ if [ "$image_build" = "all" ]; then
     discovery-ramdisk
     overcloud-control
     overcloud-compute
+    echo "Successfully built all necessary images."
 else
     eval "$image_build"
+    echo "Successfully built $image_build image."
 fi