openvpn: Add more hotplug events

openvpn: Add more hotplug events

OpenVPN supports more hooks than just 'up' and 'down'. Especially
reacting to 'route-up' and 'route-pre-down' events could be important.
When routing table changes, it can make sense to adapt firewall, run
some tests or change even more routes. This change passes those events
to hotplug, so it is easy to react to them without changing
configuration files provided by VPN provider.

Signed-off-by: Michal Hrusecky <michal.hrusecky@turris.com>