Deleting VM with 'az vm delete' does not delete its disk

If I delete a VM with _az vm delete_ I expect all dependent resources to be deleted too. But disks are not.

E.g. I have VM _tw-001-westeurope_

```
kristian@opossum:~$ az vm show -n tw-001-westeurope -g tw-rg-westeurope
{
"availabilitySet": null,
"diagnosticsProfile": null,
"hardwareProfile": {
"vmSize": "Basic_A1"
},
"id": "/subscriptions/***/resourceGroups/tw-rg-westeurope/providers/Microsoft.Compute/virtualMachines/tw-001-westeurope",
"identity": null,
"instanceView": null,
"licenseType": null,
"location": "westeurope",
"name": "tw-001-westeurope",
"networkProfile": {
"networkInterfaces": [
{
"id": "/subscriptions/***/resourceGroups/tw-rg-westeurope/providers/Microsoft.Network/networkInterfaces/tw-001-westeuropeVMNic",
"primary": null,
"resourceGroup": "tw-rg-westeurope"
}
]
},
"osProfile": {
"adminPassword": null,
"adminUsername": "azureuser",
"computerName": "tw-001-westeurope",
"customData": null,
"linuxConfiguration": {
"disablePasswordAuthentication": true,
"ssh": {
"publicKeys": [
{
"keyData": "***",
"path": "/home/azureuser/.ssh/authorized_keys"
}
]
}
},
"secrets": [],
"windowsConfiguration": null
},
"plan": null,
"provisioningState": "Succeeded",
"resourceGroup": "tw-rg-westeurope",
"resources": null,
"storageProfile": {
"dataDisks": [],
"imageReference": {
"id": "/subscriptions/***/resourceGroups/tw-rg-westeurope/providers/Microsoft.Compute/images/tw-image-2017-11-13",
"offer": null,
"publisher": null,
"resourceGroup": "tw-rg-westeurope",
"sku": null,
"version": null
},
"osDisk": {
"caching": "None",
"createOption": "FromImage",
"diskSizeGb": 30,
"encryptionSettings": null,
"image": null,
"managedDisk": {
"id": "/subscriptions/***/resourceGroups/tw-rg-westeurope/providers/Microsoft.Compute/disks/tw-001-westeurope",
"resourceGroup": "tw-rg-westeurope",
"storageAccountType": "Standard_LRS"
},
"name": "tw-001-westeurope",
"osType": "Linux",
"vhd": null
}
},
"tags": {},
"type": "Microsoft.Compute/virtualMachines",
"vmId": "abdaa320-2fef-4c22-aca1-339682f21bfd",
"zones": null
}
```

and delete it with _az vm delete_:

```
kristian@opossum:~$ az vm delete -g tw-rg-westeurope -n tw-001-westeurope --yes
{
"endTime": "2017-11-16T13:48:11.344229+00:00",
"error": null,
"name": "6a7b12f7-779d-4014-9794-720feb670f0f",
"startTime": "2017-11-16T13:47:09.907105+00:00",
"status": "Succeeded"
}
```

the disk that was attached to the VM is still there:

```
kristian@opossum:~$ az disk show -n tw-001-westeurope -g tw-rg-westeurope
{
"creationData": {
"createOption": "Copy",
"imageReference": null,
"sourceResourceId": "/subscriptions/***/resourceGroups/tw-rg-westeurope/providers/Microsoft.Compute/snapshots/tw-image-2017-11-13_envfb1jz4qg",
"sourceUri": null,
"storageAccountId": null
},
"diskSizeGb": 30,
"encryptionSettings": null,
"id": "/subscriptions/***/resourceGroups/tw-rg-westeurope/providers/Microsoft.Compute/disks/tw-001-westeurope",
"location": "westeurope",
"managedBy": null,
"name": "tw-001-westeurope",
"osType": "Linux",
"provisioningState": "Succeeded",
"resourceGroup": "tw-rg-westeurope",
"sku": {
"name": "Standard_LRS",
"tier": "Standard"
},
"tags": {},
"timeCreated": "2017-11-16T12:28:51.081460+00:00",
"type": "Microsoft.Compute/disks",
"zones": null
}
```

---

### Environment summary

Install Method (e.g. pip, interactive script, apt-get, Docker, MSI, edge build) / CLI version (`az --version`) / OS version / Shell Type (e.g. bash, cmd.exe, Bash on Windows)

```
azure-cli (2.0.20)

acr (2.0.14)
acs (2.0.18)
appservice (0.1.19)
backup (1.0.2)
batch (3.1.6)
batchai (0.1.2)
billing (0.1.6)
cdn (0.0.10)
cloud (2.0.9)
cognitiveservices (0.1.9)
command-modules-nspkg (2.0.1)
component (2.0.8)
configure (2.0.12)
consumption (0.1.6)
container (0.1.12)
core (2.0.20)
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dla (0.0.13)
dls (0.0.16)
eventgrid (0.1.5)
extension (0.0.5)
feedback (2.0.6)
find (0.2.7)
interactive (0.3.11)
iot (0.1.13)
keyvault (2.0.13)
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network (2.0.17)
nspkg (3.0.1)
profile (2.0.15)
rdbms (0.0.8)
redis (0.2.10)
resource (2.0.17)
role (2.0.14)
servicefabric (0.0.5)
sql (2.0.14)
storage (2.0.18)
vm (2.0.17)

Python location '/opt/az/bin/python3'
Extensions directory '/home/kristian/.azure/cliextensions'

Python (Linux) 3.6.1 (default, Oct 18 2017, 20:41:18)
[GCC 4.8.4]

```