More sensible staging defaults

[elharo]

Found some internal docs that we should change these flags from the defaults to true when deploying:

--enable_jar_splitting=true --enable_jar_classes=true --delete_jsps=true

Currently these default to false. It's not immediately obvious if these ever need to be false, but they shouldn't be false by default.

It might make sense to fix this in appengine-plugins-core instead.

[briandealwis]

We could incorporate these in the default appengine-web.xml template.