distro: expose default OSTreeRef()

Rather than setting this automagically, expose it to the caller. For
now the only caller we have simply passes it back in, so this is a
noop.

In follow-up commits this will be used to resolve the parent commit.

This is tested by verifying that the generated manifests do not
change.

Signed-off-by: Tom Gundersen <teg@jklm.no>

internal/distro/distro_test_common/distro_test_common.go

@@ -88,6 +88,9 @@ func TestDistro_Manifest(t *testing.T, pipelinePath string, prefix string, distr
 			got, err := imageType.Manifest(tt.ComposeRequest.Blueprint.Customizations,
 				distro.ImageOptions{
 					Size: imageType.Size(0),
+					OSTree: distro.OSTreeImageOptions{
+						Ref: imageType.OSTreeRef(),
+					},
 				},
 				repos,
 				tt.RpmMD.Packages,