destroy of meraki_appliance_site_to_site_vpn leaves configuration as-is

[joekohlsdorf]

Observed behavior

When destroying a meraki_appliance_site_to_site_vpn resource, all configuration that was previously made (Mode, Hubs, Subnets) stays as-is.

Expected behavior

Destroying the resource should disable the VPN, all configuration should be set back to the default.
The simplest way would be to set mode = 'None' but I also believe that #109 could fix this.

Reproduction steps:

1. You first need to create a valid VPN Hub, otherwise the API rejects the next step.

2. In the test network create a site-to-site VPN spoke configuration like this:

resource "meraki_appliance_site_to_site_vpn" "vpn" {
  network_id = "L_123456"

  mode = "spoke"
  hubs = [
    {
      hub_id            = "L_654321"
      use_default_route = false
    }
  ]

  subnets = [
    {
      local_subnet = "192.168.128.0/24"
      use_vpn      = true
    }
  ]
}

3. Run terraform destroy -target=meraki_appliance_site_to_site_vpn.vpn

4. Observe the configuration in the Meraki Dashboard. All configuration that was made is unchanged.