chore(validation): move functions from constants

- Move validation functions from `internal/constants` to `internal/configuration/settings/validation`
- Concatenate all OpenVPN constants in `internal/constants/openvpn.go`

internal/provider/privateinternetaccess/portforward.go

@@ -15,7 +15,7 @@ import (
 	"strings"
 	"time"
 
-	"github.com/qdm12/gluetun/internal/constants"
+	"github.com/qdm12/gluetun/internal/models"
 	"github.com/qdm12/gluetun/internal/provider/utils"
 	"github.com/qdm12/golibs/format"
 )
@@ -33,7 +33,13 @@ var (
 func (p *PIA) PortForward(ctx context.Context, client *http.Client,
 	logger utils.Logger, gateway net.IP, serverName string) (
 	port uint16, err error) {
-	server := constants.PIAServerWhereName(p.servers, serverName)
+	var server models.PIAServer
+	for _, server = range p.servers {
+		if server.ServerName == serverName {
+			break
+		}
+	}
+
 	if !server.PortForward {
 		logger.Error("The server " + serverName +
 			" (region " + server.Region + ") does not support port forwarding")