barbican: change secret store to simple crypto

Current state (as soon as !285 (merged) has been merged)

Barbican has a file-based key store, which is not suitable for production.

Desired state

Barbican has a more secure key store. Although the simple crypto plugin isn't meant to be used for production either, it is an improvement and a first step.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information