obnam config file

src/freedombone-utils-backup

     # obnam can backup multiple machines with different domain names to
     # a repository. To be able to restore directories from different
     # machines we need to enforce a single client name for all backups
-    echo "client-name = ${PROJECT_NAME}" > /etc/obnam.conf
+    echo '[config]' > /etc/obnam.conf
+    echo "client-name = ${PROJECT_NAME}" >> /etc/obnam.conf
 }
 
 function backup_directory_to_usb {