Improve SDK and Notebook user experience of ray cluster local interaction

[tedhtchang]

Investigate how to call these methods in cluster.up() because users should not need to explicitly call them.

generate_cert.generate_tls_cert(cluster_name, namespace)
generate_cert.export_env(cluster_name, namespace)

cc @MichaelClifford

[tedhtchang]

resolved by PR#100