openqa: Use default graphics backend and fix test issues

This fixes #517 (closed)

Note that the test-iso-installer-x86_64 job will fail until someone with repo maintainer permissions fixes #547 (closed)

Also, the tests fail early on with no candidate needle with tag(s) 'gnome_install_1' matched, this may be that the UI has changed and the needles need updating. It will be easier to solve once we can view tests in the webUI again.