Transform Namespace --cluster + args into a slice

[feedmeapples]

What was changed

• Changed namespace --cluster plus cluster args into a slice

Why?

previous syntax for clusters was pretty ugly. It was requiring to set --cluster to specify just one cluster name, then the args[1:] would mean the rest of the cluster (after namespace arg)

before

tctl namespace register --cluster active my-namespace standby

after

tctl namespace register --cluster active --cluster standby my-namespace  

Checklist

1. Closes

2. How was this tested:

added unit test

3. Any docs updates needed?