New-AzureRmVm/Vmss ImageName should also support VHD.

### Description

Currently, New-AzureRmVm/Vmss supports an image id as an image name. New-AzureRmVm also supports uploading VHD to Azure storage and then creating VM from the storage. It should be easy to implement support for creating VM/VMSS from VHD in Azure Storage.

URL or storage/container/blob id?

### Script/Steps for Reproduction

```powershell

```

### Module Version

```powershell
Get-Module -Name AzureRM -ListAvailable
```

### Environment Data

```powershell
$PSVersionTable
```

### Debug Output

```

```