Support offsite backups

[ctheune]

When considering a separate job type for S3 backups vs. disk backups we could also create "replication" jobs that could be integrated as separate jobs that work on other jobs and that use the distributed scheduler API to place the remote backup somewhere else.

As most of that could be a simple rsync (maybe pay attention to extended attributes) we could even create automatic temporary SSH keys in a handshake process.

Alternatively, they could be features ontop of existing jobs with a unified API for marking which jobs want to be backed up to a remote destination.