openstack: WaitForImage: treat 404 as retryable

Addresses comment at
https://github.com/mitchellh/packer/issues/1415#issuecomment-165739549

builder/openstack/step_create_image.go

@@ -69,13 +69,13 @@ func WaitForImage(client *gophercloud.ServiceClient, imageId string) error {
 		image, err := images.Get(client, imageId).Extract()
 		if err != nil {
 			errCode, ok := err.(*gophercloud.UnexpectedResponseCodeError)
-			if ok && errCode.Actual == 500 {
+			if ok && (errCode.Actual == 500 || errCode.Actual == 404) {
 				numErrors++
 				if numErrors >= maxNumErrors {
 					log.Printf("[ERROR] Maximum number of errors (%d) reached; failing with: %s", numErrors, err)
 					return err
 				}
-				log.Printf("[ERROR] 500 error received, will ignore and retry: %s", err)
+				log.Printf("[ERROR] %d error received, will ignore and retry: %s", errCode.Actual, err)
 				time.Sleep(2 * time.Second)
 				continue
 			}