Use default skuname shared Azure Disk

Shared blob disks are now created with default skuname value if the
referenced storage class possesses empty values for storageaccounttype
and skuname. Prior, the provisioning process would fail if a new
storage account needed to be created. This is because a storage account
may not be created with an empty sotrageaccounttype.

pkg/volume/azure_dd/azure_provision.go

@@ -267,7 +267,7 @@ func (p *azureDiskProvisioner) Provision(selectedNode *v1.Node, allowedTopologie
 				return nil, err
 			}
 		} else {
-			diskURI, err = diskController.CreateBlobDisk(name, storage.SkuName(storageAccountType), requestGiB)
+			diskURI, err = diskController.CreateBlobDisk(name, storage.SkuName(skuName), requestGiB)
 			if err != nil {
 				return nil, err
 			}