Fix an annoying warning generated by cron when using postgresql::backup

The cronjob created by postgresql::backup runs as root, but switches to postgres
user to dump the database. However, as the postgres user is unable to cd to
/root, the cronjob to complain. Added a "cd /tmp" to the cronjob so that the
warning is removed.

Signed-off-by: Samuli Seppänen <samuli@openvpn.net>