Add ability to configure policy_files_path and policy_files

This change adds the ability to configure policy_files_path
and policy_files configs used by the policy enforcement (RBAC)
support in Horizon.

Closes-bug: #1383458
Change-Id: Iad33e68d09b81b384d4333ef9fa785939d43f9e6
(cherry picked from commit 5ecc0eaa69175e70e14c3aaad7e0757ab32bdb02)