Add role based access control (RBAC) to Barbican

Barbican must support RBAC on its various REST methods. This commit adds logic to enforce
RBAC rules defined in a policy.json to the REST resource API methods they need to
protect for unauthorized access. The implementation utilizes the latest oslo policy
module updates to read, parse and enforce RBAC rules.

Change-Id: I94b1709532540947ab3f1c9878c883a1fd167183
Implements: blueprint rbac-add-support