Fix #195 Adding rpmBrpJavaRepackJars setting 

When activated with

```scala
rpmBrpJavaRepackJars := false
```

A  the `__os_install_post` macro will be added to the `rpmPre in Rpm` setting and jar repacking is disabled.

http://swaeku.github.io/blog/2013/08/05/how-to-disable-brp-java-repack-jars-during-rpm-build/

@timperrett can you verify this?