Unify the get_layers calls across all implements of manifest schemas to ensure we have a common type returned

Also renames some methods to make it more clear what kind of information they return

image/docker/interfaces.py

@@ -32,18 +32,18 @@ class ManifestInterface(object):
     """ Returns the bytes of the manifest. """
     pass
 
-  @abstractproperty
-  def layers(self):
-    """ Returns the layers of this manifest, from base to leaf or None if this kind of manifest
-        does not support layers. """
-    pass
-
   @abstractproperty
   def layers_compressed_size(self):
     """ Returns the total compressed size of all the layers in this manifest. Returns None if this
         cannot be computed locally.
     """
 
+  @abstractmethod
+  def get_layers(self, content_retriever):
+    """ Returns the layers of this manifest, from base to leaf or None if this kind of manifest
+        does not support layers. The layer must be of type ManifestImageLayer. """
+    pass
+
   @abstractmethod
   def get_leaf_layer_v1_image_id(self, content_retriever):
     """ Returns the Docker V1 image ID for the leaf (top) layer, if any, or None if