Afaik, there's no option available to force the use of ssl in database settings. But, when running application and dmbs on separate hosts in an untrusted network, you want to use ssl indeed.
As a workaround, you can easily setup stunnel on the application server to accept connections and proxy them over ssl to your database server.
Note: when using ssl with postgres, be sure to put appropriate ssl key/certificate pairs in place, preferably using a self-setup CA. The easyrsa helper-scripts provided with openvpn are a good start to setup an internal CA. Be sure to only trust a postgresql server which can present a certificate that is signed by your own CA.
Never just enable ssl and use the default generated key/certificates. Doing that won't help you from being attacked with a man in the middle attack on your network .