Add User Metadata to Barbican Secrets

Problem Description
===================

Users may require to add additional data to a Barbican Secret. A user may
want to add a description of the Secret as well as other necessary data
such as geolocation, rate, allowed time-access, etc. This will allow user
applications/services to check the user-metadata for certain fields in order
to allow/disallow an action to Barbican. Content can be in any format.

Proposed Change
===============

The proposed change will be to add a new attribute to Barbican
Secrets in order for user meta-data to be stored. A new API
endpoint must also be created for the manipulation of the metadata.