Some packages requirements depends on inventory variables
(`kube_proxy_mode` in that case but it could apply to others).
As the case seems pretty rare, instead of adding complexity to pkgs, we
add an escape hatch to use jinja conditions.
That should be revisited if we find ourselves shoehorning lots of logic
in this later on.
Since the structure we're setting in place for installing packages has
some complexity, add a JSON schema to avoid frustrating errors when
modifying the informations (adding/removing packages install).