Manifest.digest static method was renamed

Static method Manifest.digest was renamed to
Manfiest.calculate_digest in the mongoengine
conversion

closes #1672
https://pulp.plan.io/issues/1672

plugins/pulp_docker/plugins/registry.py

@@ -376,7 +376,7 @@ def get_manifest(self, reference):
                 msg = msg.format(e=expected_digest, d=digest)
                 raise IOError(msg)
         else:
-            digest = models.Manifest.digest(manifest)
+            digest = models.Manifest.calculate_digest(manifest)
         return digest, manifest
 
     def get_tags(self):