New commits for the html5as tutorial

All the commits taking in count new changes such as the new recipe switch-softwaretype or Eric's merge request: https://lab.nexedi.com/nexedi/slapos/merge_requests/1010.

I also added a new and final commit "html5as: switch to slapos.recipe.build" (see my MR https://lab.nexedi.com/nexedi/slapos/merge_requests/1012) after the current final one "html5as: add sla computer parameter" (current tutorial). However, I'm not sure what name should be used.

The aim of this new commit is to create an "advanced" stage in the tutorial.