HotFix: Correct document ID when updating storage status

If no ```self.pargs.flowcell```, which is the case when only updating a tarball, then ```self.pargs.flowcell == None```, which will cause ```fc_db_id = f_conn.id_view.get(self.pargs.flowcell)``` to return [this](http://tools.scilifelab.se:5984/_utils/document.html?flowcells/aec505d818c34d61b0cfe15640728ac0) document. 

This PR makes use of the ```self.pargs.tarball``` if no ```self.pargs.flowcell``` parameter is present.