OpenVPN log to file for debugging

In #805 (closed), it would be nice to have the OpenVPN log file for debugging. Code we currently have for debugging:

 if verb > 3 {
     arg = append(
         arg,
         "--log", "/tmp/leap-vpn.log")
 }

Comments

• verb can be set via env variable
• verb>3 is often too verbose, verb 3 + log file would be enough most of the time
• Writing to /tmp/leap-vpn.log probably won't work on Windows!?

Proposal

• Allow user to set openvpn log file via env variable (to enable logging, not the file name/location)
• Alow user to set openvpn verbosity via env variable (like it is currently)
• But: I know on Linux, /tmp/leap-vpn.log is written as root user. Is this a privilege escalation? => Get temp dir by Go, don't hardcode /tmp

Any opinion on this @atanarjuat ?