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/provider/pia.go

@@ -8,7 +8,7 @@ import (
 	"github.com/qdm12/gluetun/internal/models"
 )
 
-func buildPIAConf(connections []models.OpenVPNConnection, verbosity int, root bool, cipher, auth string, extras models.ExtraConfigOptions) (lines []string) {
+func buildPIAConf(connection models.OpenVPNConnection, verbosity int, root bool, cipher, auth string, extras models.ExtraConfigOptions) (lines []string) {
 	var X509CRL, certificate string
 	if extras.EncryptionPreset == constants.PIAEncryptionPresetNormal {
 		if len(cipher) == 0 {
@@ -52,7 +52,8 @@ func buildPIAConf(connections []models.OpenVPNConnection, verbosity int, root bo
 		// Modified variables
 		fmt.Sprintf("verb %d", verbosity),
 		fmt.Sprintf("auth-user-pass %s", constants.OpenVPNAuthConf),
-		fmt.Sprintf("proto %s", connections[0].Protocol),
+		fmt.Sprintf("proto %s", connection.Protocol),
+		fmt.Sprintf("remote %s %d", connection.IP, connection.Port),
 		fmt.Sprintf("cipher %s", cipher),
 		fmt.Sprintf("auth %s", auth),
 	}
@@ -62,9 +63,6 @@ func buildPIAConf(connections []models.OpenVPNConnection, verbosity int, root bo
 	if !root {
 		lines = append(lines, "user nonrootuser")
 	}
-	for _, connection := range connections {
-		lines = append(lines, fmt.Sprintf("remote %s %d", connection.IP, connection.Port))
-	}
 	lines = append(lines, []string{
 		"<crl-verify>",
 		"-----BEGIN X509 CRL-----",