Automatically use bare-bones app.yaml when users don't enter it in global WAR/JAR deploy dialog

The global WAR/JAR deploy dialog requires users to provide a WAR/JAR file and app.yaml. The minimal app.yaml contents to make it work is
```
runtime: java
env: flex
```
In some sense, requiring an explicit app.yaml makes sense, because it is the file that describes how users want to deploy their app. In another sense, however, just assuming the minimal app.yaml when not provided will increase accessibility for beginners.

Anyway, I think we will consider only the global WAR/JAR deploy dialog, because we automatically create `src/main/appengine/app.yaml` when adding flex facets to projects.

UI will need tweaks to make this happen.