Policy file paths on osfamily=RedHat have changed

On RedHat distributions of OpenStack (i.e. Fedora, RDO and RHOS),
the path of the RBAC policy files has been changed to differ from
upstream as they strive to have all configuration pieces under
/etc/openstack-dashboard instead of /usr/share/openstack-dashboard/

This change merely mimics the change Red Hat performed downstream:
https://github.com/redhat-openstack/horizon/commit/76e15a3cd419a0038b44687effec95b41e656d79

Change-Id: Ib1692244fed6191af84af0123c78129d71991a41