refactor(scrub): replace umoci logic in scrub implementation (#1845)

- implement scrub also for S3 storage by replacing umoci
- change scrub implementation for ImageIndex
- take the `Subject` into consideration when running scrub
- remove test code relying on the umoci library. Since we started
relying on images in test/data, and we create our own images using
go code we can obtain digests by other means. (cherry picked from commit 489d4e2d23c1b4e48799283f8281024bbef6123f)

Signed-off-by: Andrei Aaron <aaaron@luxoft.com>
Signed-off-by: Andreea-Lupu <andreealupu1470@yahoo.com>

errors/errors.go

@@ -50,6 +50,7 @@ var (
 	ErrCliBadConfig                   = errors.New("cli: bad config")
 	ErrRepoNotFound                   = errors.New("repository: not found")
 	ErrRepoBadVersion                 = errors.New("repository: unsupported layout version")
+	ErrRepoBadLayout                  = errors.New("repository: invalid layout")
 	ErrManifestNotFound               = errors.New("manifest: not found")
 	ErrBadManifest                    = errors.New("manifest: invalid contents")
 	ErrUploadNotFound                 = errors.New("uploads: not found")