Define a formal manifest interface and implement in the schema1 and schema2 manifests

This will allow us to pass arbitrary manifests to the model

image/docker/interfaces.py

@@ -0,0 +1,41 @@
+from abc import ABCMeta, abstractproperty
+from six import add_metaclass
+
+@add_metaclass(ABCMeta)
+class ManifestInterface(object):
+  """ Defines the interface for the various manifests types supported. """
+  @abstractproperty
+  def digest(self):
+    """ The digest of the manifest, including type prefix. """
+    pass
+
+  @abstractproperty
+  def media_type(self):
+    """ The media type of the schema. """
+    pass
+
+  @abstractproperty
+  def manifest_dict(self):
+    """ Returns the manifest as a dictionary ready to be serialized to JSON. """
+    pass
+
+  @abstractproperty
+  def bytes(self):
+    """ Returns the bytes of the manifest. """
+    pass
+
+  @abstractproperty
+  def layers(self):
+    """ Returns the layers of this manifest, from base to leaf. """
+    pass
+
+  @abstractproperty
+  def leaf_layer_v1_image_id(self):
+    """ Returns the Docker V1 image ID for the leaf (top) layer, if any, or None if none. """
+    pass
+
+  @abstractproperty
+  def blob_digests(self):
+    """ Returns an iterator over all the blob digests referenced by this manifest,
+        from base to leaf. The blob digests are strings with prefixes.
+    """