Default Vandelay import folder is not readable by opensrf

In opensrf.xml.example, under the open-ils.vandelay config, the temporary location for MARC import files is set to /tmp like so:

<importer>/tmp</importer>

On currently-supported versions of Ubuntu and Debian, this won't work, because Apache uses a private tmp folder instead. As a result, temporary MARC files get uploaded to the private tmp folder, and then Vandelay can't find the file again and gives an error like "unable to read MARC file /tmp/whatever.mrc".

We should modify opensrf.xml.example to use a folder that is accessible to Vandelay, such as /openils/var/tmp, and ensure that the folder is created during installation (either by putting a note in the docs or just going ahead and creating it). Alternatively, we could update the docs to set PrivateTmp=false in Apache systemd config.