SFOS still has QtLocation 5.2. That "osm.mapping.host" parameter was added in 5.6. Documentation for QtLocation 5.2 is not officially available, but maybe Ubuntu 15.04 might be close. But note that Jolla has backported individual commits and updated individual things like the HERE plugin, so it's understandably quite difficult to figure out.

What you're trying to do is (to my knowledge) simply not possible with the SFOS version of QtLocation. My advice would be to wait for QtLocation 5.6 (maybe we need a new community IRC pressure meeting?) or if you can't wait, ask Martin about his pure QML component. Poor Maps uses QtLocation's Map component, but has custom tile loading to work around stuff like this, but that's not designed to be reusable.