Dropped inbox_group agency for tasks in a oneclient setup.

For a one client setup the inbox_group agency for tasks is not necessary and leads to confusion because of the localroles wich are added for the inbox_group  (on the dossiers, tasks and documents). Besides the acgeny functionality in a one client setup is already given, with the local administrator functionality (see  https://github.com/4teamwork/opengever.core/blob/master/opengever/task/browser/transitioncontroller.py#L94).

Therefore I reworked the LocalRolesSetter, so that in a oneclient setup it doesn't add local roles for the inbox_group.

@lukasgraf do you have any object against?
cc @jone