feat(alerting): add nop uploader

ref #6183

conf/defaults.ini

@@ -421,7 +421,7 @@ url = https://grafana.net
 #################################### External Image Storage ##############
 [external_image_storage]
 # You can choose between (s3, webdav)
-provider = s3
+provider =
 
 [external_image_storage.s3]
 bucket_url =

conf/sample.ini

@@ -375,8 +375,8 @@
 #################################### External image storage ##########################
 [external_image_storage]
 # Used for uploading images to public servers so they can be included in slack/email messages.
-# you can choose between (s3, webdav or internal)
-;provider = s3
+# you can choose between (s3, webdav)
+;provider =
 
 [external_image_storage.s3]
 ;bucket_url =

pkg/components/imguploader/imguploader.go

@@ -10,6 +10,13 @@ type ImageUploader interface {
 	Upload(path string) (string, error)
 }
 
+type NopImageUploader struct {
+}
+
+func (NopImageUploader) Upload(path string) (string, error) {
+	return "", nil
+}
+
 func NewImageUploader() (ImageUploader, error) {
 
 	switch setting.ImageUploadProvider {
@@ -53,5 +60,5 @@ func NewImageUploader() (ImageUploader, error) {
 		return NewWebdavImageUploader(url, username, password)
 	}
 
-	return nil, fmt.Errorf("could not find specified provider")
+	return NopImageUploader{}, nil
 }