fix kubeconfig context name generation for regional use (0.8.x)

[montmanu]

the recent changes to support regional clusters did not include updates to the kubeconfig context name generation logic.. some sample results of the current issue when used with the regional parameter:

kubectl config set-context gke_my-project__my-cluster --namespace qa
# Context "gke_my-project__my-cluster" created.

the namespace resource is successfully created, and if you explicitly configure the namespace of resources within your templates, those resources will be configured properly, but for users that do not explicitly configure the namespace within their templates (relying on the context instead), their resources will end up applied, but to the default namespace.

[montmanu]

an alternative approach might be to implement the equivalent of

kubectl config set-context $(kubectl config current-context) --namespace foo

[tonglil]

Thanks for the fix @montmanu!