Optimize which OpenVPN instances are restarted 

An OpenVPN instance is only restarted if:
- the new highest-priority interface for the instance is different from the one the instance is currently running on; or
- the OpenVPN instance is currently running on the interface that has gone down or up; or
- the OpenVPN instance should be running on the interface that has gone down or up.
With this change, you can have a configuration like:
  Gateway group WANpriority - WAN Tier 1, OPT1 Tier 2
  OpenVPN instance on WANpriority
OpenVPN is normally binding to the WAN IP.
OPT1 interface goes down and/or up - the OpenVPN instance continues happily running on WAN without being restarted.