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.

从 stackablectl 的 CLI 入口点开始，检查命令行参数的定义方式以及 Kubernetes 上下文的选择方式。使用 issue 中的命令示例对作为验收检查；当每条命令都能在不更改 kubectl 默认上下文的情况下针对其指定的集群，并且选项名称已经确定时，即视为完成。