[metaticket] build the openvpn command string to be invoked
need a function to get the openvpn command with all the needed options/flags.
by now, we should at least implement a switch to:
- launch management interface over uds on darwin/linux
- over telnet on win.
stub for:
- front / daemon (watch --debug flag)
(from redmine: created on 2012-06-28, closed on 2012-09-20, relates #356 (closed), relates #382 (closed), relates #383 (closed))