store_aip: correctly reference XML elements
store_aip: correctly reference XML elements

store_aip was still referencing the old, accidentally non-namespaced
METS elements, so it could no longer find the correct children.

refs #5159

artefactual/archivematica-storage-service,artefactual/archivematica-storage-service,artefactual/archivematica-storage-service,artefactual/archivematica-storage-service