Use slapos.tool.networkcache extension by default. This extension requires you define network-cache to make it work. If network-cache is not defined, the previous behaviour for download is preserved.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@42540 20353a03-c40f-0410-a6d1-a30d3c3de9de