added autocompletion for vcluster names & namespaces in vclusterctl.

[Ygshvr]

What issue type does this pull request address? (keep at least one, remove the others) /kind enhancement

What does this pull request do? Which issues does it resolve? (use resolves #<issue_number> if possible) resolves #673

Please provide a short message that should be published in the vcluster release notes added autocompletion for vcluster names & namespaces in vclusterctl

What else do we need to know? With this change the autocompletion will also work with --namespace flag (giving all the namespaces in the suggestion) and delete, connect, pause, resume and upgrade sub-commands (giving all the vclusters in the suggestion).