issue with Standard deployment: user experience

[ludoch]

Now that the Maven plugin supports both Flex and Standard, it might be confusing to move back and forth between a Flex runtime (that needs an app.yaml) and a Standard runtime (Java7 or coming Java8).
If the user just adds runtime:java7 in the app.yaml, it does not work for Standard which currently mandates an appengine-web.xml used by the prod runtime...

It would be nice to detect early this configuration error in the plugin (and Gradle And intellig): Standard (i.e no vm:true nor env:flex) should not be currently defined via app.yaml but with an appengine-web.xml

Medium term, they are discussions about opening support for non staging phase in standard and use user defined app.yaml, but this is not currently what the product does or what is documented, so we should add the check in the new plugins....
Now, it might be complicated to detected a generated app.yaml via staging compared to a user provided app.yaml, but it can be done (the old gcloud plugin had some logic there already)