When multus is run inside a container, eg as a daemonset, we can use
InClusterConfig() to automatically get the service account token, the
CA certificate, and the apiserver address, all without a kubeconfig
file.
Plugin went away, replaced by a minimal Spec.Config field so that the CNI
version the thick plugin supports could be indicated. This means we need
to also inject the network name into the minimal CNI JSON config when
loading the Spec.Config if the network name isn't there already.
