Fix unexpected behaviour when `kubeconfig` is set (#296)

As detailed in https://github.com/k3s-io/k3s-ansible/issues/295

, this
commit fixes the issue that if `kubeconfig` is set to anything other
than the defaults value, then:

- `~/.kube/config` is modified.
- No file at `{{ kubeconfig }}` is created.
- Any existing file and `{{ kubeconfig }}` is deleted.

Signed-off-by: Nick To <nick@nickto.net>