Minikube: Warn if incompatible kubectl version is in use

[tstromberg]

Currently, it's possible to use minikube on a host running v1.6.1 against a master running v1.12.1. This isn't a supported configuration, as per https://github.com/kubernetes/community/blob/master/contributors/design-proposals/release/versioning.md#supported-releases-and-component-skew

We should have a flight check that validates that ancient version of kubectl aren't in use. This would also serve as an early warning to users about issues such as #3203

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[tstromberg]

If someone wants to implement this:

• showKubectlConnectInfo in cmd/minikube/cmd/start.go might be the right place to start
• kubectl version --client -o json will return an easily JSON blob containing the current version of the kubectl client.
• If clientVersion is more than one minor release away from k8sVersion, issue a warning, like:

out.WarningT("The version of kubectl [{{.kcVersion}}] installed is incompatible with your Kubernetes [{{.k8sVersion}}] deployment"). Please upgrade/downgrade as necessary, or useminikube kubectlto connect to your cluster

[nanikjava]

Assigning this ticket to myself

[nanikjava]

/assign @nanikjava

[tstromberg]

@nanikjava - excellent news! Please let me know if you need any additional help, either here or on the Slack #minikube channel.

[nanikjava]

Output from 'kubectl version --output=json' is applicable for all version of kubectl.

Following is a sample output

{
  "clientVersion": {
    "major": "1",
    "minor": "12",
    "gitVersion": "v1.12.0",
    "gitCommit": "0ed33881dc4355495f623c6f22e7dd0b7632b7c0",
    "gitTreeState": "clean",
    "buildDate": "2018-09-27T17:05:32Z",
    "goVersion": "go1.10.4",
    "compiler": "gc",
    "platform": "linux/amd64"
  },
  "serverVersion": {
    "major": "1",
    "minor": "16",
    "gitVersion": "v1.16.0",
    "gitCommit": "2bd9643cee5b3b3a5ecbd3af49d09018f0773c77",
    "gitTreeState": "clean",
    "buildDate": "2019-09-18T14:27:17Z",
    "goVersion": "go1.12.9",
    "compiler": "gc",
    "platform": "linux/amd64"
  }
}

[nanikjava]

@tstromberg PR ready https://github.com/kubernetes/minikube/pull/5596

Thank you.