Add 'remove_older_than' param

Duplicity resources should have a `remove_older_than` param to remove
backups that are older than a pre-set value.

The value is set to 30D (30 days) to ensure that we:
 - are consistent across repos, and
 - are regularly taking full backups instead of relying on incrementals over
   longer periods of time
Merge pull request #239 from alphagov/duplicity-remove-older-than-30d

Add 'remove_older_than' param to Duplicity resources