Feature Request: 让vxnet支持managerIP参数

希望可以增加manager_ip参数来自定义默认网关地址

### Terraform Configuration Files
```hcl
resource "qingcloud_vxnet" "private_network"{
  name = "private_network"
  type = 1
  vpc_id = "${qingcloud_vpc.vpc.id}"
  ip_network = "172.22.1.0/24"
  manager_ip = "172.22.1.2"
}

```

https://github.com/yunify/qingcloud-sdk-go/blob/master/service/router.go#L886
