openvpn: let instances drop to nobody in default config.

This is for security precautions.  As persist_tun and persist_key are
already there, this should not cause compatibility issue.

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45961 3c298f89-4303-0410-b956-a3cf2f4a3e73