updated description of sku name values (#3467)

* updated description of sku name values

* added default sku name value

Author: tfitzmac

specification/compute/resource-manager/Microsoft.Compute/stable/2018-06-01/compute.json

@@ -5909,7 +5909,7 @@
       "properties": {
         "name": {
           "type": "string",
-          "description": "The sku name."
+          "description": "The sku name. Possible values are: **Aligned** for managed disks, and **Classic** for unmanaged disks. Default value is Classic, if not specified."
         },
         "tier": {
           "type": "string",