PAR2 not working with --archive-dir switch (lp:#1353066)

[Original report](https://bugs.launchpad.net/bugs/1353066) created by **holmz12 (holmz12-deactivatedaccount)**
```
I've upgraded to 0.6.24 and thought I would enable the new par2 wrapper
backend. Unfortunately I think I've found a bug with it. My setup uses
--archive-dir=somedir to place the cache files in a different location.
With this it appears that duplicity only creates par2 files for the main
volumes and fails to create a par2 file for either the signatures file or
the manifest file. The result of this is when I try and do an incremental
backup it says no such file when it tries to read the par2 signatures file.

Timestamps of these files taken from when I was playing with this back in
May, but the -v9 log below is from now.

Without that switch:

duplicity-full-signatures.20140527T171029Z.sigtar.gpg
duplicity-full-signatures.20140527T171029Z.sigtar.gpg.par2
duplicity-full-signatures.20140527T171029Z.sigtar.gpg.vol000+188.par2
duplicity-full.20140527T171029Z.manifest.gpg
duplicity-full.20140527T171029Z.manifest.gpg.par2
duplicity-full.20140527T171029Z.manifest.gpg.vol00+38.par2
duplicity-full.20140527T171029Z.vol1.difftar.gpg
duplicity-full.20140527T171029Z.vol1.difftar.gpg.par2
duplicity-full.20140527T171029Z.vol1.difftar.gpg.vol000+198.par2

With that switch:

duplicity-full-signatures.20140527T170909Z.sigtar.gpg
duplicity-full.20140527T170909Z.manifest.gpg
duplicity-full.20140527T170909Z.vol1.difftar.gpg
duplicity-full.20140527T170909Z.vol1.difftar.gpg.par2
duplicity-full.20140527T170909Z.vol1.difftar.gpg.vol000+198.par2

Duplicity version 0.6.24, Python 2.7.8, FreeBSD 10.0-STABLE.

My command line:

duplicity full -v9 --allow-source-mismatch --archive-dir=archive
--asynchronous-upload --encrypt-key=$ENCRYPTKEY --include-globbing-
filelist=filelist --gpg-options="--cipher-algo=AES256 --digest-algo=SHA512
--compress-algo=bzip2 --bzip2-compress-level=9" --name=dropbox --sign-
key=$SIGNKEY --volsize=30 / par2+file:///root/backup/dropbox/Backup

The -v9 log shows that it doesn't attempt to write the .par2 files:

Making directory archive/dropbox/duplicity_temp.1
Create Par2 recovery files
Deleting archive/dropbox/duplicity_temp.1/duplicity-full-
signatures.20140805T185
955Z.sigtar.gpg
Writing /root/backup/dropbox/Backup/duplicity-full-
signatures.20140805T185955Z.s
igtar.gpg
Deleting tree archive/dropbox/duplicity_temp.1
Selecting archive/dropbox/duplicity_temp.1
Deleting archive/dropbox/duplicity_temp.1
Deleting archive/dropbox/duplicity-full-
signatures.20140805T185955Z.sigtar.gpg
Making directory archive/dropbox/duplicity_temp.1
Create Par2 recovery files
Deleting archive/dropbox/duplicity_temp.1/duplicity-
full.20140805T185955Z.manife
st.gpg
Writing /root/backup/dropbox/Backup/duplicity-
full.20140805T185955Z.manifest.gpg
Deleting tree archive/dropbox/duplicity_temp.1
Selecting archive/dropbox/duplicity_temp.1
Deleting archive/dropbox/duplicity_temp.1
Deleting archive/dropbox/duplicity-full.20140805T185955Z.manifest.gpg

Making directory /tmp/duplicity-fVl4Qm-tempdir/duplicity_temp.1
[Errno 2] No such file or directory:
'/root/backup/dropbox/Backup/duplicity-ful$
.20140805T185955Z.manifest.gpg.par2'
Releasing lockfile <LinkLockFile:
'/root/backup/archive/dropbox/blah.example.com-1c0$
400.82804' -- 'archive/dropbox/lockfile'>
Removing still remembered temporary file /tmp/duplicity-fVl4Qm-
tempdir/mktemp-LS
aj4A-2
Removing still remembered temporary file /tmp/duplicity-fVl4Qm-
tempdir/mkstemp-S
Of3Zr-1
Cleanup of temporary directory /tmp/duplicity-fVl4Qm-tempdir failed - this
is pr
obably a bug.
```
Original tags: archive-dir par2

Story points: 6.0