'az image create' doesn't provide a way to create premium storage images

[gregjhogan]

az image create seems to always create Standard_LRS images (even if the source VM/disk is Premium_LRS)

example:

az image create --resource-group $VM_RESOURCE_GROUP --name $IMAGE_NAME --source $VM_NAME --os-type Linux

or

az image create --resource-group $VM_RESOURCE_GROUP --name $IMAGE_NAME --source $VM_OS_DISK_NAME --os-type Linux

resource created:

{
  "name": "ubuntu-16.04-base-image-ssd",
  "provisioningState": "Succeeded",
  "sourceVirtualMachine": null,
  "storageProfile": {
    "dataDisks": [],
    "osDisk": {
      "blobUri": null,
      "caching": "None",
      "diskSizeGb": 30,
      "osState": "Generalized",
      "osType": "Linux",
      "snapshot": null,
      "storageAccountType": "Standard_LRS"
    },
    "zoneResilient": null
  },
  "tags": {},
  "type": "Microsoft.Compute/images"
}

We need to be able to pass the storage account type for disks, or default to the storage account type of the source disk
https://docs.microsoft.com/en-us/rest/api/compute/images/createorupdate#storageaccounttypes