CI/kubevirt: allow every vars in kubevirt template to be overriden

The current templating of kubevirt VirtualMachine relies on global
ansible variables, except for the group the nodes are meant to be in.

In order to have more flexibility (in particular, mixed OS cluster for
instances), expect now an abitrary  dict to be passed to the template ;
this allows to embed directly in the nodes definition any variable used
by the template.