Tenant Creation Should initialize some content

When creating a new tenant; we need to initialize some content like the com.nuxeo.brand.management.dataset.operation.InitSampledTenantOperation operation.

It requires some refactoring; what I suggest (which are open to discussion):

• Move/Rename TestHelper to main scope
• Remove test-jar generation, and dependency in assembly.xml and test-dataset; as it only requires the TestHelper
• Extract resources/Randomizer/whatever from test-dataset to a new module, dataset for instance - to prevent us from having pictures/etc in the core module. (Just a personal preference)
• Add this new module to the package by default
• Extract document creation part of com.nuxeo.brand.management.dataset.DataSetInjectorImpl#initSampledTenant to a new helper in this new module; and create a listener on Domain creation to add the new content.
• Add a test to verify that it works

Accptance;

• When calling Domain.CreateNew, I see some demo content.