Add output parameter to the docker-env none shell

[afbjorklund]

$ ./out/minikube docker-env --shell none --output json | jq
{
  "DOCKER_CERT_PATH": "/home/anders/.minikube/certs",
  "DOCKER_HOST": "tcp://192.168.49.2:2376",
  "DOCKER_TLS_VERIFY": "1",
  "MINIKUBE_ACTIVE_DOCKERD": "minikube"
}

Closes #12242

The test is actually longer than the code, due to the complexity in comparing maps and arrays in formats without sorting.

[k8s-ci-robot]

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