Support flex in App Engine content block

[briandealwis]

The App Engine content block should support App Engine flexible.

We'll need descriptor models for the various yaml files.

[briandealwis]

app.yaml descriptions:

1. app.yaml (App Engine standard environment for Python)
2. Configuring your App with app.yaml  (App Engine flexible environment for Java)
3. appengine-web.xml Reference (App Engine standard environment for Java)