Update generate_sample.sh to allow custom configs

If any *.conf.sample files exist in the tools/config/
directory we append those to the generated script as well.

This allows projects to append custom config file snippets to
the generated config file and is useful for things like
keystone_authtoken information.

Change-Id: I2af0db4d5c47fc307540cdb0d1cce171d3aa5b27