This allows subnets to optionally be created with a description.

For example:

```
    subnets = [
        {
            subnet_name           = "subnet-01"
            subnet_ip             = "10.10.10.0/24"
            subnet_region         = "us-west1"
        },
        {
            subnet_name           = "subnet-02"
            subnet_ip             = "10.10.20.0/24"
            subnet_region         = "us-west1"
            subnet_private_access = "true"
            subnet_flow_logs      = "true"
            description           = "This subnet has a description"
        },
```

This reverts commit 0784627.