Address PR feedback by adding missing sizes and clarifying comment

docs/properties.md

@@ -36,6 +36,9 @@ Specifies VM compute characteristics
 * Values:
     * `Standard_DC2s`
     * `Standard_DC4s`
+    * `Standard_DC1s_v2`
+    * `Standard_DC2s_v2`
+    * `Standard_DC4s_v2`
     * `Standard_DC8_v2`
 
 Refer to the [VM sizes in Azure](https://docs.microsoft.com/en-us/azure/virtual-machines/windows/sizes) for more details.
@@ -119,4 +122,4 @@ Alternatively, you can specify image source URL.
 * Path: `properties/{linux|windows}Profile/osImage/url`
 * Value: string
 
-Note that newer VMs, such as `Standard_DC8_v2` sizes, require a `gen2` image.
+Note that the _v2 version of the DC series VMs require a gen2 image of the OS. For example, in the case of Ubuntu, "18_04-lts-gen2".

pkg/api/const.go

@@ -50,6 +50,9 @@ var AllowedLocations = []string{
 var AllowedVMSizes = []string{
 	"Standard_DC2s",
 	"Standard_DC4s",
+	"Standard_DC1s_v2",
+	"Standard_DC2s_v2",
+	"Standard_DC4s_v2",
 	"Standard_DC8_v2",
 	"Standard_D2s_v3",
 	"Standard_D4s_v3",