KUBERNETES=libvirt-coreos cluster/kube-up.sh produced the following error:

cluster/../cluster/libvirt-coreos/../../cluster/common.sh: line 83: user_args[@]: unbound variable

This was coming from the fact that, as a libvirt-coreos cluster runs locally on local VMs,
there is no authentication mechanism. This led to have user_args of common.sh unset.

In the case of libvirt-coreos, it is in fact expected to have no authentication token.