Duplicity: Correctly handle Dropbox access token [has patch]

Currently, Backupninja does not have a way of specifying the Dropbox access token for Duplicity. This never used to be an issue as the access token used to be hard-coded in Duplicity's dpbxbackend.py file, but it was modified to accept the access token as an environment variable instead.

Seeing as I can't clone backupninja on this site, here's the patch: dropbox-patch.diff. This patch updates Backupninja's Duplicity integration to pass the correct environment variables.