update duplicity.cnf template

templates/duplicity.cnf.j2

@@ -1,3 +1,5 @@
+{% if duplicity_backend == s3 -%}
 export PASSPHRASE="{{ duplicity_s3_passphrase }}"
 export AWS_ACCESS_KEY_ID={{ duplicity_s3_access_key }}
 export AWS_SECRET_ACCESS_KEY={{ duplicity_s3_secret_key }}
+{% endif %}