Fix & feat: Cyberghost server groups

- Allow multiple comma separated values for CYBERGHOST_GROUP
- Defaults to all UDP groups
- If TCP is enabled, defaults to all TCP groups
- Check groups specified match the protocol
- Default Cyberghost group to empty
- Adjust formatting and messages

internal/configuration/openvpn_test.go

@@ -33,7 +33,7 @@ func Test_OpenVPN_JSON(t *testing.T) {
     "server_selection": {
       "tcp": false,
       "regions": null,
-      "group": "",
+      "groups": null,
       "countries": null,
       "cities": null,
       "hostnames": null,