This will provide a Castellan based secret store, which will
allow secret stores which have a castellan backend to be used
behind barbican. The initial example of this is the Vault
backend.
Unit tests have been added. In local tests,
most of the functional tests do in fact pass with a local Vault
backend, though this will need to be demonstrated with a later
review which establishes a Vault based gate.
Change-Id: Ib30fb79304014592bfc37938839d60a4c10c244d
89cb777941