Fix AWS Node Labels Error

Now the `kubespray-aws-inventory.py` script always set a node_labels key
to ansible_host.

When AWS instance did not set property labels, it would be an empty
string.

The TASK `Write kubelet config file (kubeadm or non-kubeadm)` will
failed with a msg:

`AnsibleUndefinedVariable: 'unicode object' has no attribute 'items'`.