Can't restore old backup that hasn't been updated for a long time (lp:#1097849)

[Original report](https://bugs.launchpad.net/bugs/1097849) created by **Carlo Fragni (carlofragni)**
```
I am trying to restore a backup from an old server that has been
deactivated. The last time backup was executed was over six months ago. The
backup files do exist but duplicity doesn't seem to find any backup chains
because the last backup signature file dates from a long time ago. Using
ubuntu 12.04 amd 64, python 2.7.3, duplicity  0.6.18 . Here is the -v9
output:

$ duplicity -v9 --no-encryption --allow-source-mismatch
s3+http://<S3_NAME>/ ~/restore/

Using archive dir:
/home/<USER>/.cache/duplicity/8d63fa05952285d6243341c30b9ad3b6
Using backup name: 8d63fa05952285d6243341c30b9ad3b6
Import of duplicity.backends.botobackend Succeeded
Import of duplicity.backends.gdocsbackend Succeeded
Import of duplicity.backends.u1backend Succeeded
Import of duplicity.backends.tahoebackend Succeeded
Import of duplicity.backends.ftpsbackend Succeeded
Import of duplicity.backends.imapbackend Succeeded
Import of duplicity.backends.sshbackend Succeeded
Import of duplicity.backends.ftpbackend Succeeded
Import of duplicity.backends.hsibackend Succeeded
Import of duplicity.backends.localbackend Succeeded
Import of duplicity.backends.webdavbackend Succeeded
Import of duplicity.backends.cloudfilesbackend Succeeded
Import of duplicity.backends.rsyncbackend Succeeded
Main action: restore
================================================================================
duplicity 0.6.18 (February 29, 2012)
Args: /usr/bin/duplicity -v9 --no-encryption --allow-source-mismatch
s3+http://<S3_NAME>/ /home/<USER>/restore/
Linux <COMPUTER_NAME> 3.2.0-35-generic #55-Ubuntu SMP Wed Dec 5 17:42:16
UTC 2012 x86_64     x86_64
/usr/bin/python 2.7.3 (default, Aug  1 2012, 05:14:39)
[GCC 4.6.3]
================================================================================
Using temporary directory /tmp/duplicity-YSboMf-tempdir
Registering (mkstemp) temporary file /tmp/duplicity-YSboMf-tempdir/mkstemp-
AVob8O-1
Temp has 480694202368 available, backup will use approx 34078720.
Local and Remote metadata are synchronized, no sync needed.
0 files exist on backend
0 files exist in cache
Extracting backup chains from list of files: []
Last full backup date: none
Collection Status
-----------------
Connecting with backend: BotoBackend
Archive dir: /home/<USER>/.cache/duplicity/8d63fa05952285d6243341c30b9ad3b6

Found 0 secondary backup chains.
No backup chains with active signatures found
No orphaned or incomplete backup sets found.
Removing still remembered temporary file /tmp/duplicity-YSboMf-
tempdir/mkstemp-AVob8O-1
Traceback (most recent call last):
  File "/usr/bin/duplicity", line 1403, in <module>
    with_tempdir(main)
  File "/usr/bin/duplicity", line 1396, in with_tempdir
    fn()
  File "/usr/bin/duplicity", line 1330, in main
    restore(col_stats)
  File "/usr/bin/duplicity", line 623, in restore
    restore_get_patched_rop_iter(col_stats)):
  File "/usr/bin/duplicity", line 645, in restore_get_patched_rop_iter
    backup_chain = col_stats.get_backup_chain_at_time(time)
  File "/usr/local/lib/python2.7/dist-packages/duplicity/collections.py",
line 952, in     get_backup_chain_at_time
    raise CollectionsError("No backup chains found")
CollectionsError: No backup chains found
```

Story points: 6.0