Update schema2 builder to take media type

Modify manifest builder so it can be used to build
manifests with different configuration media types.
Rename config media type const to image config.

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>
Signed-off-by: Derek McGowan <derek@mcgstyle.net>

testutil/manifests.go

@@ -73,7 +73,7 @@ func MakeSchema1Manifest(digests []digest.Digest) (distribution.Manifest, error)
 func MakeSchema2Manifest(repository distribution.Repository, digests []digest.Digest) (distribution.Manifest, error) {
 	ctx := context.Background()
 	blobStore := repository.Blobs(ctx)
-	builder := schema2.NewManifestBuilder(blobStore, []byte{})
+	builder := schema2.NewManifestBuilder(blobStore, schema2.MediaTypeImageConfig, []byte{})
 	for _, digest := range digests {
 		builder.AppendReference(distribution.Descriptor{Digest: digest})
 	}