Add example startup script (#1146)

* Create openapi-appengine.yaml

* Add comment to openapi.yaml about deploying on App Engine.
Remove openapi-appengine.yaml.

* Add query string authentication option to Endpoints clients.

* Fix bracket line spacing.

* Add openapi-appengine.yaml file for App Engine deployments.

* Remove alternative auth scheme.

* Change cloud.goog to appspot.com

* Add simple IAP app for GCE.

* Add hostname to example app's response.

* Update IAP example backend and requirements.

* Remove unnecessary '/healthz' endpoint, move constants after imports.

* Fix lint

* Add example startup scripts.

Author: buserp

iap/example_startup_script.sh

@@ -0,0 +1,11 @@
+apt-get -y install git
+apt-get -y install virtualenv
+git clone https://github.com/GoogleCloudPlatform/python-docs-samples
+cd python-docs-samples/iap
+virtualenv venv
+source venv/bin/activate
+pip install -r requirements.txt
+cat example_gce_backend.py |
+  sed -e "s/YOUR_BACKEND_SERVICE_ID/$(gcloud compute backend-services describe my-backend-service --global --format="value(id)")/g" |
+  sed -e "s/YOUR_PROJECT_ID/$(gcloud config get-value account | tr -cd "[0-9]")/g" > real_backend.py
+gunicorn real_backend:app -b 0.0.0.0:80