Add disable environment override option.

[dpersson]

Sometimes you don't want to overwrite the provided arguments with possible settings from your environment variables.
This adds a flag to convert-kubeconfig for disabling environment override. It uses the flag in "kubelogin get-token" UpdateFromEnv() to return early and disregard possible setting of environment variables.

kubelogin convert-kubeconfig -l spn --client-id xxx --client-secret xxx --disable-environment-override

will result in:

{
  "name": "clusterxxx",
  "user": {
    "exec": {
      "apiVersion": "client.authentication.k8s.io/v1beta1",
      "args": [
        "get-token",
        "--login",
        "spn",
        "--server-id",
        "xxx",
        "--client-id",
        "***",
        "--tenant-id",
        "***",
        "--environment",
        "AzurePublicCloud",
        "--client-secret",
        "xxx",
        "--disable-environment-override"
      ],
      "command": "kubelogin",
      "env": null,
      "installHint": "\nkubelogin is not installed which is required to connect to AAD enabled cluster.\n\nTo learn more, please go to https://aka.ms/aks/kubelogin\n",
      "provideClusterInfo": false
    }
  }
}

Provides solution for #196

[dpersson]

@microsoft-github-policy-service agree

@microsoft-github-policy-service agree

[weinong]

LGTM