Feature: stackablectl support for kubectl contexts

I would like to use stackablectl against multiple clusters without having to change the default kubectl context.

For example, I can do:

kubectl --context cluster-a get pods -A
kubectl --context clister-b get pods -A

I would like to be able to do:

stackablectl --context cluster-a list stacklets
stackablectl --context cluster-b list stacklets

An alternative argument could be --kube-context in case --context is misleading, or it could mean something for stackablectl itself.

從 stackablectl 的 CLI 進入點開始，檢查命令列引數的定義方式，以及 Kubernetes context 的選取方式。使用 issue 中的命令範例對作為驗收檢查；當每個命令都能在不變更 kubectl 預設 context 的情況下針對其指定的叢集，且選項名稱已確定時，即表示完成。