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/utils.go

@@ -2,11 +2,15 @@ package provider
 
 import (
 	"context"
+	"math/rand"
 	"time"
 
+	"github.com/qdm12/gluetun/internal/models"
 	"github.com/qdm12/golibs/logging"
 )
 
+type timeNowFunc func() time.Time
+
 func tryUntilSuccessful(ctx context.Context, logger logging.Logger, fn func() error) {
 	const retryPeriod = 10 * time.Second
 	for {
@@ -27,3 +31,7 @@ func tryUntilSuccessful(ctx context.Context, logger logging.Logger, fn func() er
 		}
 	}
 }
+
+func pickRandomConnection(connections []models.OpenVPNConnection, source rand.Source) models.OpenVPNConnection {
+	return connections[rand.New(source).Intn(len(connections))] //nolint:gosec
+}