Add Purchase Plan to Azure Machine Provider Spec

With Azure marketplace images, some images require that the
subscription accept the terms of a purchase plan before using; and
that purchase plan information must be included in Azure API calls
when provisioning a VM. Images that do not require purchase plans
must not submit purchase plan info.

This adds a PurchasePlanName field to designate when a purchase
plan should be generated for an image.

machine/v1beta1/types_azureprovider.go

@@ -174,6 +174,13 @@ type Image struct {
 	Version string `json:"version"`
 	// ResourceID specifies an image to use by ID
 	ResourceID string `json:"resourceID"`
+	// PurchasePlanName is the name of the purchase plan for a third-party marketplace image.
+	// If the image requires a purchase plan, this value should equal the SKU. If the image
+	// does not require a purchase plan, this field must be empty. Purchase plans must already
+	// be accepted in the subscription. For more information see:
+	// https://docs.microsoft.com/en-us/azure/virtual-machines/linux/cli-ps-findimage#check-the-purchase-plan-information
+	// +optional
+	PurchasePlanName string `json:"purchasePlanName,omitempty"`
 }
 
 type OSDisk struct {