backup.pl: Fixes bug#13404 - Clear out OpenVPN certs before doing restore

backup.pl: Fixes bug#13404 - Clear out OpenVPN certs before doing restore

- Existing situation is if four new client connections are created and then it is decided
   to restore to an earlier stage the new certficates will be in the certs directory but
   not usable from the WUI page as they are no longer shown in the client connection table
   as that now shows the ones from the restored backup.
- This patch clears the /var/ipfire/ovpn/certs/ directory before restoring the contents
   of the backup so that the certs directory only holds what was in the backup.

Fixes: Bug#13404
Tested-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>