Refactor buildNSXVPC

[TaoZou1]

currently buildNSXVPC also includes logic to check if vpc has been changed.
It's better to check if vpc changed then call buildNSXVPC like:
if vpcNeedUpdated {
createdVpc, err := buildNSXVPC(...)
}

it also uses:
if createdVpc == nil {
log.Info("no VPC changes detect, skip creating or updating process")
return existingVPC[0], nil
}

it's better to use vpcNeedUpdated instead of createdVpc == nil