Update init-gke.sh script with external bootstrap

[wajihyassine]

Name and Version

charts/*

What is the problem this feature will solve?

Lack of instructions around setting up external access

What is the feature you are proposing to solve the problem?

Automating the steps in this guide.

Specifically allowing users a flag such as --setup-external-access and will run gcloud commands to create IP addresses, dns records, setup a domain if not already setup and creation of a oauth2 client for Turbinia authentication

What alternatives have you considered?

Having this in a document of instructions instead