The parameter 'backup' in backup-create is never read by our Trove API[1], so
I guess it's a extra key argument for history reason. In addition, the
'backup-copy' subcommand should be removed as well because it does not
support in current Trove API.
[1] https://github.com/openstack/trove/blob/master/trove/backup/service.py#L61-L77
Change-Id: I33d439e0037efd4573eeae186758fd778ba5c514