wrongly set mountpoint in qiita.data_directory when different roots in settings

[antgonza]

The original code, see below, assumes that qc.working_dir and qc.base_data_dir have the same "root", so when they don't it creates a "weird" base. This happened in qiita-rc: ('/a/b/working_directory/', '/c/d/'), repestibly.

            # When creating a type is expected that a new mountpoint is created
            # for that type, note that we are going to check if there is an
            # extra path for the mountpoint, which is useful for the test
            # environment
            qc = qiita_config
            mp = relpath(qc.working_dir, qc.base_data_dir).replace(
                'working_dir', '')
            mp = mp + name if mp != '/' and mp != '' else name
            sql = """INSERT INTO qiita.data_directory
                        (data_type, mountpoint, subdirectory, active)
                        VALUES (%s, %s, %s, %s)"""
            qdb.sql_connection.TRN.add(sql, [name, mp, True, True])