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