Add secret store plugin page

Add a Sphinx document page detailing the secret store plugin. Also add
cross references between this page and the crypto plugin page.

Change-Id: I59f66440eb5d247197ac47dd0994b44b4a9eec4b
Add certificate plugin page

Add a page detailing the certificate plugin, including classes and
the interaction of plugin methods with barbican 'core' logic.

Change-Id: Iade34e4b021fe12afb7e4eeba107327cbb70e43f
Smoke tests for secrets in Barbican Functional Tests

Also updated secret models and behaviors for a more usable api.
Modified client to create models using updated model API.

Change-Id: I414f4869e1013caec0a2e6c69e4e176aba4e43e7
Merge "Add secret store plugin page"
Merge "Add certificate plugin page"
Merge "Smoke tests for secrets in Barbican Functional Tests"