In quickstarts/custom provide sample configuration for Karaf 'minimal' installation

In the quickstarts/custom demo (which shows how to create a Karaf custom assembly), it would be good if we could provide a working sample configuration corresponding to the old Karaf 'minimal' installation and provide a test for this configuration.

This issue originated as a documentation request, FUSEDOC-1074, but it does not really make sense to document a configuration without having a corresponding test in place. So, I think it makes more sense to enhance the quickstart code with this sample configuration.