duplicity shouldn't sync so much

[Martin Nowak]

Too many commands of duplicity first sync manifests and signatures to the local cache, even though they could just work with remote data.
Since signature files can become multiple GB, this is fairly annoying, e.g. when checking collection-status from a dev machine or before removing old backups.
Syncing should only be done when it's necessary, or alternatively there should be at least an option to skip syncing.