Fix omero.data.dir ignored in current develop branch (see #10828)

Some folks have been waiting for this, so here's the PR.

To test, run the server locally and set ``omero.data.dir`` to anything else than ``/OMERO``. Also remove the ``/OMERO`` directory before starting up the server. Everything should start nicely and data should land in the ``omero.data.dir`` configured folder.