Single connection written to openvpn configuration (#258)

- From now only a single OpenVPN connection is written to the OpenVPN configuration file
- If multiple connections are matched given the user parameters (i.e. city, region), it is picked at pseudo random using the current time as the pseudo random seed.
- Not relying on Openvpn picking a random remote address, may refer to #229 
- Program is aware of which connection is to be used, in order to use its matching CN for port forwarding TLS verification with PIA v4 servers, see #236 
- Simplified firewall mechanisms

internal/firewall/enable.go

@@ -85,8 +85,8 @@ func (c *configurator) enable(ctx context.Context) (err error) { //nolint:gocogn
 	if err = c.acceptEstablishedRelatedTraffic(ctx, remove); err != nil {
 		return fmt.Errorf("cannot enable firewall: %w", err)
 	}
-	for _, conn := range c.vpnConnections {
-		if err = c.acceptOutputTrafficToVPN(ctx, c.defaultInterface, conn, remove); err != nil {
+	if c.vpnConnection.IP != nil {
+		if err = c.acceptOutputTrafficToVPN(ctx, c.defaultInterface, c.vpnConnection, remove); err != nil {
 			return fmt.Errorf("cannot enable firewall: %w", err)
 		}
 	}