This patch adds the api functions for user metadata and tests for
those functions. It is the second of several patches which will
complete the "User Defined Metadata for Barbican Secrets" blueprint.
Other Patches will include:
1.) Documentation
Implements: blueprint add-user-metadata
Change-Id: Iaf83bf5a3b9ed2ea22d4cd3e83ba3a0d5d087adf