Default values for packageArchetype.java_server

Setting `Upstart` as `serverLoading` and `root` for `daemonUser` as default values for `packageArchetype.java_server`.

This can be changed with this

```scala
serverLoading in Debian := com.typesafe.sbt.packager.archetypes.ServerLoader.Upstart

daemonUser in Debian := "root"
```