Solve quoting issue with "su" (debian #683731) (upstream #4019)

A fix for debian #677410 on commit 29dcdbae introduces a new issue on wheezy
machines where some rsync_options gets out of the rsync command quotes when
invoked by su.

As reported, running backupninja through `bash -x` shows that the `--delete`
option from the rsync command is not included in the single quote for the `su
-c` command:

    nice -n 0 su -c '/usr/bin/rsync -av' --delete '--recursive
    --delete-excluded    //home/ /tmp/test//home//'

This patch attempt to solve that issue by using the intermediary variable
$command.