Add an option to have PersistentKeepalive setting in the generated conf files.

[berinaniesh]

Some of us use this script not to route traffic through a different server, but to link machines without static IPs. Without persistent-keepalive packets, the connection tends to sleep (at least with NetworkManager) and we can't ssh into the machine anymore if the connection is idle for some time. This can be fixed by adding the PersistentKeepalive setting in the generated conf file or adding persistent-keepalive in nmconnection files, but it would be great if there is a setting in the script to do that automatically.