This patch set adds validation logic to document_validation.py (in
Deckhand's engine module) so that components that rely on
Deckhand's engine for document rendering (such as Promenade
or Pegleg) can fail fast when they provide Deckhand with a
duplicate document. Must pass pre_validate=True to layering
module which currently is the case for Promenade, et. al.
Before this change, Deckand only supported this logic at
the DB level (requiring service instantion); this is now no longer
the case.
Change-Id: I6d1c8214775aa0f3b5efb1049972cf847f74585b
2c4c5a9c63