Proposal: New rule for kubernetes clusters on Azure CNI

[tspearconquest]

According to the Microsoft Docs for Azure CNI, when configuring a cluster to use Azure CNI, you need to create 2 subnets in your virtual network, and assign one to the cluster via vnet_subnet_id, and the other to the in-cluster service network via service_cidr.

It is a mistake to configure a cluster with only 1 subnet that and then configure a smaller CIDR block from that same subnet as the service_cidr, however tflint does not currently highlight this issue and azure does not generate an error or warning message.

Configuring a single subnet for a cluster with Azure CNI will work only under a limited (and diminishing) set of circumstances, and it is not supported by Microsoft. Since service_cidr is only set at cluster creation, and cannot be changed afterward, it would be great if this issue can be highlighted by tflint.