Add example for existing GKE cluster

[hongchaodeng]

Example terraform.tfvars

google_region     = "..."
google_project_id = "..."

anyscale_deploy_env = "test"
anyscale_org_id     = "..."

existing_vpc_name            = "..."
existing_subnet_name         = "..."
customer_ingress_cidr_ranges = "0.0.0.0/0"

gke_endpoint       = "..."
gke_ca_certificate = "..."

[brent-anyscale]

Let's merge this as is for now, but can you move the ingress-nginx helm chart into /modules/anyscale-k8s-helm and tie it to which cloud provider is in use?