Refactor plugin API methods

* Supported method now must respond to an enumeration
* Modified encrypt/generate_data_encryption_key to use supports
* Refactor a bunch of confusing kek_meta_tenant, kek_metadata,
* kek_meta_dto things
* Modify plugins to support new method signatures in abstract base class
* Unit tests for more error conditions
* Hacking fixes (but many more needed...)

Change-Id: I33c20959f145f1c8f22725f62fb410033080b64f
Implements: blueprint crypto-refine-is-supported