Add e2e test for endpoint sample

[tmatsuo]

[tmatsuo]

Some tricky parts;

• you need to use gcloud beta app deploy, maybe we should tweak our deploytrait to make the command configurable
• you need to create an API key like curl -vv -d '{"message":"foo"}' -H 'Content-Type: application/json' "${ENDPOINTS_HOST}/echo?key=${ENDPOINTS_KEY}"
• ideally you should test it without the API key and check if it's correctly 404

[bshaffer]

This was added here for App Engine Flex, but we now need e2e for GCE and GKE as well.

[jeffmendoza]

Awesome, I'm working on a doc for GKE testing.