Feature: stackablectl support for kubectl contexts

[NickLarsenNZ]

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.

[NickLarsenNZ]

Shell completion is a must, so you can do:

stackablectl --context clus<TAB>
stackablectl --context cluster-<TAB>
cluster-a
cluster-b

same as kubectl

[NickLarsenNZ]

This would come in handy for spinning up multiple Replicated clusters, and running tests against each in parallel.