gRPC on GKE tutorial: Python client (bookstore_client.py) cannot connect to the API backend in latest version of grc.io/endpointsv2/python-grpc-bookstore-server

## In which file did you encounter the issue?

https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/endpoints/bookstore-grpc/bookstore_server.py

### Did you change the file? If so, how?

Overrode the PORT env variable to set it to 8000. Changed https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/endpoints/kubernetes/grpc-bookstore.yaml

> image: gcr.io/process-mining-gke/bookstore-grpc-server:latest
> env:
> - name: PORT
> value: "8000"

## Describe the issue

This issue is with gRPC on GKE tutorial : https://cloud.google.com/endpoints/docs/grpc/get-started-kubernetes-engine

The python client (bookstore_client.py) can no longer connect to the API backend as the latest docker image build of the API backend (grc.io/endpointsv2/python-grpc-bookstore-server) ends up listening on the port specified by the PORT variable while the proxy (esp) forwards request to port 8000 as specified in the Kubernetes manifest file .

This is likely due to the changes made in https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/endpoints/bookstore-grpc/bookstore_server.py to make it work in Cloud Run which requires that the server listen on the port specified by the PORT environment.