Valid configs fail if fields missing

[goddenrich]

eg. get

kube_config.py", line 587, in load_config
 self._merge(item, config[item], path)
KeyError: 'clusters'

for the example config from the k8s documentation https://kubernetes.io/docs/tasks/access-application-cluster/configure-access-multiple-clusters/

contexts:
- context:
 cluster: development
 namespace: frontend
 user: developer
 name: dev-frontend
- context:
 cluster: development
 namespace: ramp
 user: developer
 name: dev-ramp-up
- context:
 cluster: development
 namespace: storage
 user: developer
 name: dev-storage
- context:
 cluster: scratch
 namespace: default
 user: experimenter
 name: exp-scratch