Stub Swift Space

refs #6938

Add Swift Space with all functions stubbed out.  Add dependencies; remove
explicit six dependency, since it's required by several packages already,
and upgrade bagit so it uses the same version of six as the swiftclient
packages.
Minor fixes

Set all SS logs to use storage_service.log.  Set default
Space.staging_path to /var/archivematica/storage_service/
wip