Update glbc deletion and check_http_status. Add OAuth and kubectl con…

…text helper.

* Extract glbc resource names from ingress object since default-backend
  backend service and NEG cannot be identified based on ingress name.
* Add get_or_create_oauth_brand and get_oauth_client for getting OAuth
  client id and secret.
* Add --context option to kubectl commands so functions can be run when
  multiple clusters are created, and a helper function to get the test
  context.
* Add an option for check_http_status to skip ssl verification.