make subnet_id be optional and name be updatable for vip

[ShiChangkuo]

the testing result as follows:

$ make testacc TEST='./huaweicloud' TESTARGS='-run  TestAccNetworkingV2VIP_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud -v -run  TestAccNetworkingV2VIP_basic -timeout 360m
=== RUN   TestAccNetworkingV2VIP_basic
--- PASS: TestAccNetworkingV2VIP_basic (90.47s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud       90.522s