sandbox: Fix path to Sb DB socket for ovn-controller-vtep.

There was a race between
commit ff4439d ("Build OVN using external OVS directory") and
commit 381c931 ("ovn-ic: Interconnection controller with AZ registeration.")
Both touched sandbox code, but were accepted in the oppisite order
to one they were prepared and sent to the mail list.

This led to the case where new code from the first commit wasn't
updated by the second one leaving ovn-controller-vtep with
incorrect Southbound DB socket configuration, so it doesn't
work inside the sandbox:

  sandbox/ovn-controller-vtep.log:
  |reconnect|INFO|unix:ovn/tutorial/sandbox/ovnsb_db.sock:
        connection attempt failed (No such file or directory)

Fixes: 381c931 ("ovn-ic: Interconnection controller with AZ registeration.")
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>
Acked-by: Han Zhou <hzhou@ovn.org>
Signed-off-by: Numan Siddique <numans@ovn.org>

tutorial/ovs-sandbox

@@ -626,7 +626,7 @@ done
 rungdb $gdb_ovn_controller_vtep $gdb_ovn_controller_vtep_ex \
     ovn-controller-vtep --detach --no-chdir --pidfile -vconsole:off \
     $OVN_CTRLR_PKI --log-file -vsyslog:off \
-    --ovnsb-db=unix:"$sandbox"/ovnsb_db.sock
+    --ovnsb-db="$OVN_SB_DB"
 
 cat <<EOF