Update glbc deletion and check_http_status. Add OAuth and kubectl context helper.

[sawsa307]

• 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.