Rename `create` to `register` for clusters registration in rok CLI

Rework rok CLI and rename create to register for cluster registration.

• The rok CLI command rok kube cluster used the word create but no actual Kubernetes cluster was created here. Instead, Krake is made aware that the Kubernetes cluster exists, through its kubeconfig file.
• Hence the word register better describes what is going to do and makes a rok CLI more intuitive
• register is syntactic sugar for API POST request that register a cluster that has been already created