Be able to disable target OS for some tests

[Skunnyk]

It would be cool to be able to disable some OS on specific tests.
For example, xfce4-volumed-pulse or xfce4-pulseaudio-plugin can't be build on DragonFlyBSD because there is no pulseaudio support anymore (see http://lists.dragonflybsd.org/pipermail/users/2016-August/313010.html ).

Maybe by using {obj:xxx} variable reference ? ( https://docs.openstack.org/infra/jenkins-job-builder/definition.html#variable-references )