Refactoring adding irods user to project

irods user property is implemented as DM property type Array.

For some unknown reasons that DM's support for Array property is broken.

This change, overwrite on_post method in project handler, is basically
bring that default DM behaviour back.

This will reduce i commands execution to 1 per new irods user.

refs #1792 LW hangs for individual users and logs shows Timeout Error.