upload/azure: rename Image to PageBlob

The UploadImage method doesn't actually create an image. It creates a Page
Blob. Blob is something like S3 object but in the Azure terminology. Page
Blob means that's optimized for random access and it's the only blob type
that can be used to create images.

This commit cleans up the terminology so it's less confusing.

Signed-off-by: Ondřej Budai <ondrej@budai.cz>

cmd/osbuild-worker/jobimpl-osbuild.go

@@ -234,13 +234,13 @@ func (impl *OSBuildJobImpl) Run(job worker.Job) error {
 				continue
 			}
 
-			metadata := azure.ImageMetadata{
+			metadata := azure.BlobMetadata{
 				ContainerName: options.Container,
-				ImageName:     t.ImageName,
+				BlobName:      t.ImageName,
 			}
 
 			const azureMaxUploadGoroutines = 4
-			err = azureStorageClient.UploadImage(
+			err = azureStorageClient.UploadPageBlob(
 				metadata,
 				path.Join(outputDirectory, options.Filename),
 				azureMaxUploadGoroutines,