The export_dir is now mandatory

Every calculator writes on the file system, so the export_dir must be mandatory. We must also check upfront that the directory exists and is writeable. See https://bugs.launchpad.net/oq-engine/+bug/1207248.
The tests are running here https://ci.openquake.org/job/zdevel_oq-commonlib/199 and here https://ci.openquake.org/job/zdevel_oq-engine/815.