ovpn: bug fix for $OLDGW with multiple routers

Currently, vpn-up.sh failed when multiple routers is available. e.g. when both wired and wireless network is available.The problem is caused by $OLDGW will contain two lines begin with `via`.In the difference of commit 37d256f6c3bce85d50caa7883c50a9c3e7916de6, there is an extra  `| head -n1` handling this problem but it has been removed.From the discussion with wzyboy, It will be more graceful to use `| grep -v metric` to solve this problem. And this is just what this commit do,.