STI save-artifacts script should be optional during build time

[davidpelaez]

I was puzzled trying to build a new image for STI in that it kept failing about not finding the scripts. This was in a nginx static file server where incremental builds make no sense, hence I didn't create a save-artifacts script. This made the build command failed because it encountered a http 404.

The save-artifacts script should be optional because it doesn't always apply opposed to run and assemble.