Add order plugin metadata entity and logic

To support certificate plugin workflows, plugins needs a means to persist
their state in between method invocations by Barbican core. This CR adds
that persistence feature, by adding a new SQLAlchemy model and migration file,
and a new repository.

Change-Id: Ic0f74d49ab8c97e5b15c61fbc8c64d00bd9c8b5e
Implements: blueprint add-ssl-ca-support
Merge "Add order plugin metadata entity and logic"