[Josh merged some subsequent commits:]

* Guard via idempotency against termination between migration operations
* Final corrections and tweaks
* Pass passphrase through to all duplicity calls

Empirical evidence (a failed cron job) shows that cleanup requires the
passphrase (so it presumably needs to decrypt metadata), and though
remove-older-than has been working fine without it, it won't do any harm
to set it in case that changes or there are any special cases.

* Add back the archive-dir override but locate it at STORAGE_ROOT/backup/cache