This is my first time trying to use juju to install a service, and I have not yet found a charm to install openvpn-server easily (with sensible configuration). Is any such charm available, and how can I use it?
No there is currently no OpenVPN charm (yet!).
There's an open bug report for OpenVPN here:
To see if a service has a charm available you can just use the search box on http://jujucharms.com/