This patchset moves retrieval of encrypted documents to the
Deckhand controller so that components like Pegleg and
Promenade can consume the Deckhand engine offline without
running into Barbican errors.
Components can pass in `encryption_sources` to Deckhand's
rendering module which Deckhand will now use instead to resolve
secret references.
`encryption_sources` is a dictionary that maps the reference
contained in the destination document's data section to the
actual unecrypted data. If encrypting data with Barbican, the
reference will be a Barbican secret reference.
Change-Id: I1a457d3bd37101d73a28882845c2ce74ac09fdf4
039f9830da