Skip layering for control documents

This is to skip layering for control documents (those whose
metadata.schema starts with "deckhand/Control") as these documents
consist of ValidationPolicy or LayeringPolicy documents -- and
it would be both nonsensical and scary to try to layer
these types of documents.

Documentation for this will be updated during a larger overhaul
effort to improve Deckhand's documentation.

Change-Id: Ia785e54c4e26a4158b6bdc89da8b96b4455f7b39

deckhand/engine/document_wrapper.py

@@ -33,6 +33,10 @@ class DocumentDict(dict):
         return utils.jsonpath_parse(
             self, 'metadata.layeringDefinition.abstract') is True
 
+    @property
+    def is_control(self):
+        return self.metadata.get('schema', '').startswith('deckhand/Control')
+
     @property
     def schema(self):
         return self.get('schema') or ''

deckhand/engine/layering.py

@@ -469,6 +469,9 @@ class DocumentLayering(object):
             in both the parent and child documents being layered together.
         """
         for doc in self._sorted_documents:
+            # Control documents don't need to be layered.
+            if doc.is_control:
+                continue
 
             if doc.parent_selector:
                 parent_meta = self._parents.get((doc.schema, doc.name))