Add OS:Barbican:Secret resource

This adds a Barbican Secret resource to contrib plugins allowing secrets
to be created using barbican's infrastructure.

Co-Authored-By: Anderson Mesquita <andersonvom@gmail.com>
Implements: blueprint barbican-resources
Change-Id: Ie9d345b8a1186cd9e00f6a083652f4f0466e94c3
Merge "Add OS:Barbican:Secret resource"