Introduce ability to skip DW when no flavor is installed

In order to execute functional tests inside an xwiki docker container, I need to install only what's needed by the tests (as we're doing when we generate the hsqldb/jetty packaging on the fly for func tests). I've developed a maven plugin to install extensions in xwiki. However the DW still shows up.

So I'll need a configuration option so that the DW doesn't appear if there's no flavor installed in XWiki.