consider to include AZURE_CONFIG_DIR in convert-kubeconfig sub-command

[weinong]

when a user setups the az cli outside the home directory, e.g. in a jenkins env, the azurecli login in kubelogin will not be able to find the appropriate az context until AZURE_CONFIG_DIR is set. The idea is to enable convert-kubeconfig sub command to take this path and update kubeconfig so that get-token sub-command knows where the az context is

[VijitaLekharaju]

This will be very useful for us, and probably other customers as well, reducing the overhead for carrying around the az-context to every kubectl command executed per AKS cluster in automation scripts, shared libraries or other CI/CD implementations.

[weinong]

doc update is in #270