Adds store_secret_supports to secret_store

This adds a new method, store_secret_supports, to the SecretStore interface.
Given a KeySpec, this method will allow the plugin manager to check if a plugin
supports storing the secret data.

For example, a plugin that only supports storing secrets if the secret
attributes are defined and valid will check the attributes are not None, but
some plugins may be able to store any secret as a blob whether or not the
attributes are defined and thus will define this method to always return True.

Implements: blueprint create-secret-store
(https://blueprints.launchpad.net/barbican/+spec/create-secret-store)
Change-Id: I5f1b196f36d02587cd35aac7cdfa1c152743aaa6