Fixed examplecontent setup

Right now the examplecontent wrongly creates two repositoryroot and activate the tree portlet for the wrong one.

To fix that problem i changed the default of the `repository_root_id` for the `PathFromReferenceNumber` blueprint. I guess it's the best option, because it makes no sense to have a numbered repositoryroot by default. 

@lukasgraf what you think? or should I only adjust the repository_root_id for the examplecontent repository import configuration?