kfctl.sh delete doesn't check/change kubectl context before issuing delete

I don't think kfctl.sh delete verifies that kubectl is pointing at the right cluster before it issues a delete.

We should add a check and verify that we are pointing at the same cluster as the ksonnet app before deleting the namespace.

Here's some code to try to do that in bash.
https://github.com/kubeflow/testing/blob/master/deployment/redeploy_app.sh#L84