Make entities immutable and create proper update methods that state by signature what can be updated.

[kaizimmerm]

Another task from #197

Entities are now immutable (setters removed) Entity Update/Create builders ensure that only supported fields can be changed/set.

DSL create entity Example:

entityFactory.distributionSet().create().name("name").version("version")
                .description("desc").type(123L).modules(modules)
                .requiredMigrationStep(true);

As part of the refactoring I also migrated more test data generation into the central test data factory.

The new builders add code overhead of course. I played around with Lombok but I think the introduction of that one should be discussed independent of this pull request.