provider/openstack: Allow resizing when Flavor Name changes

Previously, resizing would only work if the flavor_id changed and
would create an error if the flavor_name changes. This commit fixes
this behavior.

It also quickly refactors the getFlavorID function to use
Gophercloud's IDFromName function. getFlavorID was the basis of
IDFromName so the exact same code is used.

Fixes hashicorp#5780

builtin/providers/openstack/resource_openstack_compute_instance_v2.go

@@ -725,12 +725,20 @@ func resourceComputeInstanceV2Update(d *schema.ResourceData, meta interface{}) e
 	}
 
 	if d.HasChange("flavor_id") || d.HasChange("flavor_name") {
-		flavorId, err := getFlavorID(computeClient, d)
-		if err != nil {
-			return err
+		var newFlavorId string
+		var err error
+		if d.HasChange("flavor_id") {
+			newFlavorId = d.Get("flavor_id").(string)
+		} else {
+			newFlavorName := d.Get("flavor_name").(string)
+			newFlavorId, err = flavors.IDFromName(computeClient, newFlavorName)
+			if err != nil {
+				return err
+			}
 		}
+
 		resizeOpts := &servers.ResizeOpts{
-			FlavorRef: flavorId,
+			FlavorRef: newFlavorId,
 		}
 		log.Printf("[DEBUG] Resize configuration: %#v", resizeOpts)
 		err = servers.Resize(computeClient, d.Id(), resizeOpts).ExtractErr()
@@ -1258,35 +1266,8 @@ func getFlavorID(client *gophercloud.ServiceClient, d *schema.ResourceData) (str
 		return flavorId, nil
 	}
 
-	flavorCount := 0
 	flavorName := d.Get("flavor_name").(string)
-	if flavorName != "" {
-		pager := flavors.ListDetail(client, nil)
-		pager.EachPage(func(page pagination.Page) (bool, error) {
-			flavorList, err := flavors.ExtractFlavors(page)
-			if err != nil {
-				return false, err
-			}
-
-			for _, f := range flavorList {
-				if f.Name == flavorName {
-					flavorCount++
-					flavorId = f.ID
-				}
-			}
-			return true, nil
-		})
-
-		switch flavorCount {
-		case 0:
-			return "", fmt.Errorf("Unable to find flavor: %s", flavorName)
-		case 1:
-			return flavorId, nil
-		default:
-			return "", fmt.Errorf("Found %d flavors matching %s", flavorCount, flavorName)
-		}
-	}
-	return "", fmt.Errorf("Neither a flavor ID nor a flavor name were able to be determined.")
+	return flavors.IDFromName(client, flavorName)
 }
 
 func resourceComputeVolumeAttachmentHash(v interface{}) int {