Rename top level registry interface to namespace

Registry is intended to be used as a repository service than an abstract collection of repositories. Namespace better describes a collection of repositories retrievable by name.
The registry service serves any repository in the global scope.

Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan)

registry/storage/manifeststore_test.go

@@ -21,7 +21,7 @@ import (
 type manifestStoreTestEnv struct {
 	ctx        context.Context
 	driver     driver.StorageDriver
-	registry   distribution.Registry
+	registry   distribution.Namespace
 	repository distribution.Repository
 	name       string
 	tag        string