The slapos default configuration decision:
- text/html does not have Last-Modified
  Because we can not calculate proper last-modified date when the page consists
  of several parts. (e.g.: Gadgets)
- resouces have 'Cache-Control: public'
  Because several versions of Firefox request 'public' to cache resources
  over SSL.
So web_checker.cfg respects the slapos default configuration decision.

- Cache-Control:public,max-age= is also a valid configuration.
- application/font resources do not need to care about Vary header.

This reverts commit 1ea46ea8.

Don't use squash because in this way we will lost the history.

Add several parameters for web_checker recipe template.