Engine implementation for document replacement

This adds support for document replacement to the
Deckhand engine _only_ for the following scenarios
_only_:

  * generic case (a replaces b, returns a only)
  * substitution case (a replaces b, c substitutes from a instead)

TODO:

  * layering case (a replaces b, c layers with a instead)
  * Modify Document unique constraint to work with
    (schema, name, layer) throughout all of Deckhand
    (including controllers, database models, and anywhere
     else as needed)

Change-Id: Ie2cea2a49ba3b9ebc42706fbe1060d94db2e5daa

deckhand/errors.py

@@ -261,6 +261,22 @@ class MissingDocumentPattern(DeckhandException):
     code = 400
 
 
+class InvalidDocumentReplacement(DeckhandException):
+    """The document replacement is invalid.
+
+    **Troubleshoot:**
+
+    * Check that the replacement document has the same ``schema`` and
+      ``metadata.name`` as the document it replaces.
+    * Check that the document with ``replacement: true`` has a parent.
+    * Check that the document replacement isn't being replaced by another
+      document. Only one level of replacement is permitted.
+    """
+    msg_fmt = ("Replacement document [%(schema)s, %(layer)s] %(name)s is "
+               "invalid. Reason: %(reason)s")
+    code = 400
+
+
 class UnsupportedActionMethod(DeckhandException):
     """The action is not in the list of supported methods.