Deployer Specific Secret Metadata

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

Deployers may require to add additional data to a Barbican Secret, which
users cannot access/modify. Currently only user metadata can be used which
allows the user to edit the metadata as well.

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

The proposed change will be to add a new list value to the barbican configuration specifying which metadata keys can only be edited by a deployer, regardless of the policy.

A new "metadata" command will be added to "barbican-manage" which will allow a deployer to perform a change to secret user metadata.