Xpk default namespace (#28)

* namespace argument support

* remove unneccesary ()

* make sure kubectl commands run in the default namespace

* all kubectl commands will run with the default namespace

xpk.py

@@ -1005,7 +1005,7 @@ def set_cluster_command(args) -> int:
   command = (
       'gcloud container clusters get-credentials'
       f' {args.cluster} --region={zone_to_region(args.zone)} --project={args.project} &&'
-      ' kubectl config view'
+      ' kubectl config view && kubectl config set-context --current --namespace=default'
   )
   return_code = run_command_with_updates(
       command, 'Set Cluster', args, verbose=False